Research Article
23 March 2012

Novel High-Rank Phylogenetic Lineages within a Sulfur Spring (Zodletone Spring, Oklahoma), Revealed Using a Combined Pyrosequencing-Sanger Approach

ABSTRACT

The utilization of high-throughput sequencing technologies in 16S rRNA gene-based diversity surveys has indicated that within most ecosystems, a significant fraction of the community could not be assigned to known microbial phyla. Accurate determination of the phylogenetic affiliation of such sequences is difficult due to the short-read-length output of currently available high-throughput technologies. This fraction could harbor multiple novel phylogenetic lineages that have so far escaped detection. Here we describe our efforts in accurate assessment of the novelty and phylogenetic affiliation of selected unclassified lineages within a pyrosequencing data set generated from source sediments of Zodletone Spring, a sulfide- and sulfur-rich spring in southwestern Oklahoma. Lineage-specific forward primers were designed for 78 putatively novel lineages identified within the pyrosequencing data set, and representative nearly full-length small-subunit (SSU) rRNA gene sequences were obtained by pairing those primers with reverse universal bacterial primers. Of the 78 lineages tested, amplifiable products were obtained for 52, 32 of which had at least one nearly full-length sequence that was representative of the lineage targeted. Analysis of phylogenetic affiliation of the obtained Sanger sequences identified 5 novel candidate phyla and 10 novel candidate classes (within Fibrobacteres, Planctomycetes, and candidate phyla BRC1, GN12, TM6, TM7, LD1, WS2, and GN06) in the data set, in addition to multiple novel orders and families. The discovery of multiple novel phyla within a pilot study of a single ecosystem clearly shows the potential of the approach in identifying novel diversities within the rare biosphere.

INTRODUCTION

The utilization of high-throughput short-read sequencing technologies in 16S rRNA-based diversity surveys (32) allowed an exponential increase in the sampling depths and number of samples analyzed for a fraction of the cost of Sanger sequencing, leading to what has been described as renaissance for the16S rRNA-based diversity analysis (71). Pyrosequencing- and, more recently, Illumina-based surveys have provided insights into spatiotemporal variations of microbial communities in various habitats as well as into the impact of various environmental factors on microbial community structure, composition, and diversity (2, 23, 24, 26, 46, 49, 60, 61). Efforts aimed at standardization of amplification and sequencing procedures (51), quality control (40, 47, 64, 79), and development of microbial community analysis programs tailored for high-throughput sequencing surveys (8, 11, 28) have resulted in the widespread democratization and adaptation of these approaches by the majority of microbial ecologists (4, 7, 9, 10, 43, 45, 5355, 78, 80).
One of the important observations obtained from currently published high-throughput diversity surveys is that a significant fraction of the obtained sequences (for example, 32% in reference 80, 13% in reference 49, 10% in reference 74, 25% to 47% in reference 36, and 15% in reference 66) is often unclassifiable; i.e., the sequence similarity to the closest classifiable relative in databases is lower than a certain preset sequence similarity threshold, e.g., 80%. A fraction of sequences belonging to such unclassifiable fractions could putatively be representatives of truly novel high-rank taxonomic lineages, where the depth achieved by high-throughput surveys allowed access to hitherto untapped reservoirs of diversity. Indeed, a significant fraction of unclassifiable sequences often belonged to rare operational taxonomic units (OTUs) (see, for example, references 5 and 24). On the other hand, a fraction of these sequences could putatively be unclassifiable due to the constraints of sequence technology (i.e., short read lengths of approximately 250 and 450 bp in FLX and Titanium pyrosequencing technology, respectively [1, 72]) and sequence analysis (i.e., classification based on identifying the nearest neighbor in databases, rather than detailed phylogenetic analysis). The relative contribution of each of these two possible scenarios to the overall proportion of unclassified sequences in a specific data set is unclear and probably depends on multiple factors such as the level of diversity in an ecosystem, length of amplicons amplified, region of 16S rRNA gene amplified, alignment and classification tool and database utilized, and set thresholds for classification.
Given the unprecedented scope of the high-throughput sequencing surveys that have been conducted (2, 23, 24, 26, 27, 46, 49, 60, 61), or are currently being conducted or are planned to be conducted in the near future (25), gauging an accurate estimate of the nature and depth of novel diversities within various ecosystems is crucial. Here, using sediments harboring an extremely diverse microbial community from Zodletone Spring, a sulfide- and sulfur-rich spring in southwestern Oklahoma, we developed and tested an approach for the experimental evaluation of the phylogenetic affiliation of selected rare members of a microbial community. This report demonstrates for the first time the feasibility of conducting an ecosystem-wide evaluation of the phylogenetic affiliations of a large number of unclassified lineages from a pyrosequencing data set. The proposed approach combines the high-throughput capabilities of pyrosequencing and the read length and phylogenetic resolution of Sanger sequencing. Our results indicate that, while a large fraction of the lineages could accurately be binned into existing lineages (classes, orders, or families) once representative longer sequences are obtained, several of the unclassifiable lineages represent novel, previously undocumented bacterial phyla and classes, hence clearly demonstrating that the rare biosphere is indeed a yet-untapped reservoir of novel phylogenetic diversity.

MATERIALS AND METHODS

Site description and sampling.

Sediments were collected from the source area of Zodletone Spring, a sulfide- and sulfur-rich spring in southwestern Oklahoma, in May 2009. Details of the spring geochemistry (18, 67, 80) have been described before. The source of the spring is a contained area (1 m2) in which anaerobic, biomass-laden, and sulfide-rich black viscous sediments are covered by an anoxic, sulfide-rich (8.4 mM) 40-cm-deep water column (18, 67). The sampling procedure has been described before (80). Sediments were stored undisturbed on ice for approximately 3 h until transferred to the laboratory, where they were stored at −20°C for 3 days prior to DNA extraction.

Pyrosequencing and identification of putatively novel lineages within the pyrosequencing data set.

Pyrosequencing, sequence analysis, binning, and alignment, as well as various aspects of the phylogenetic diversity and community structure of the pyrosequencing data set, have been detailed in a previous publication (80). Briefly, DNA was extracted from 4 adjacent (within 1 mm of each other) samples (1 g each) by the use of a FastDNA spin kit for soil (MP Biomedicals, Solon, OH). The V1 and V2 regions of the 16S rRNA gene were amplified using primer pair 8F and 338R. While this pair is not the best choice for maximizing phylogenetic capability for identification (51), or for the most accurate species richness estimates (21, 79), the choice was aimed at maximizing the size of the product obtained when paired with a universal reverse primer (see below). Overall, pyrosequencing yielded a total of 292,130 high-quality reads (83.5% of all reads obtained), with an average read length of 263 ± 17 bases. Using the Greengenes alignment and classification system, 32% of sequences (50,675 sequences in 13,619 OTUs0.03) remained unclassified using a criterion of less than 85% similarity to a closest relative in Greengenes database (80). These unclassified sequences formed 7,884, 7,339, and 6,189 distinct lineages at 8% (putative family), 10% (putative order), and 15% (putative class) sequence divergence cutoffs, respectively (13, 14).
Our current study utilized 78 such lineages for detailed pyrosequencing-Sanger analysis. These lineages were chosen by insertion of representative OTUs0.03 of the unclassified sequences (13,619 OTUs) into the ARB program (52) for preliminary phylogenetic inferences. ARB putatively grouped the majority of sequences into established higher taxonomic cutoffs, i.e., previously known divisions and candidate divisions. The remaining sequences (2,014 sequences binned into 385 OTUs and 78 lineages) were considered putatively novel and chosen for further analysis. Therefore, it is important to note that this effort represented a pilot study that did not target all novel groups in Zodletone Spring. Moreover, the census of the bacterial community in Zodletone Spring is far from complete. The pyrosequencing data set utilized in this study fails to identify all sequences within Zodletone Spring, as shown by the rarefaction curve and coverage analysis (80). Relative abundances of chosen lineages ranged between 0.0003% and 0.3% of the total pyrosequencing data set.
Sequences that were considered of poor quality as evaluated using the quality control procedures employed with the original pyrosequencing data set were removed from the data set based on the following criteria: average quality score of 25 as suggested before (see reference 39), an incorrect primer sequence, one or more ambiguous bases, a homopolymer stretch longer than 8 bases, and/or length shorter than 80 bp (80). However, recently, newer approaches have been implemented to reduce the effect of sequencing errors on downstream analysis (62) and to improve the process of quality filtering of pyrosequences. To avoid any effect that pyrosequences with suboptimal quality might have on the downstream process of identifying novel lineages, we applied the new quality-filtering approach (using a minimum average quality score of 35 over a window of 50 bases instead of an average quality score of 25 as previously applied to the entire pyrosequencing data set), as suggested in reference 62, on the 2,014 potentially novel sequences. While the more stringent approach removed 22 additional sequences and hence decreased the number to 1,992 high-quality pyrosequences, the number of OTUs0.03 and lineages did not change. This is due to the fact that the trimmed sequences belonged to OTUs with two or more sequences and that other sequences in the same OTU were of high quality. The 78 putatively novel lineages (now composed of 1,992 high-quality pyrosequences) were subsequently targeted in an effort to obtain nearly full-length sequences representative of such lineages.

Designing lineage-specific primers.

Short pyrosequences from each lineage were used to design forward lineage-specific primers by the use of the probe design function of ARB (52), with selected criteria of a length of 18 bp and a GC content within the 35% to 65% range. ARB-generated primers were further evaluated for specificity using the Probebase program of RDP (11) and through BlastN comparisons (42). Primers were designed so that they would have a difference of at least 1 bp from the closest database relative, provided that no more than 10 sequences of various affiliations with such criteria were present in public databases, and at least two mismatches to large members of all coherent phylogenetic lineages. In general, 1 to 3 forward lineage-specific primers were designed for each lineage, such that a PCR product of at least 1,100 bp would be obtained upon amplification. A list of all primers designed is shown in Table S1 in the supplemental material.

PCR amplification, sequencing, and assembly.

For each lineage, a specific forward primer was paired with a reverse universal bacterial primer, 1391R (5′-GACGGGCGGTGWGTRCA-3′), 1492R (5′-GGTTACCTTGTTACGACTT-3′), or 1525R (5′-AAGGAGGTGWTCCARCC-3′). The choice of the reverse primer to pair with the specific forward primer was based on GC content and the absence of primer dimers or other secondary structures. If no product was obtained using the first-choice reverse primer, we proceeded to the second-choice primer and so on. PCRs were conducted in a 25-μl volume. Each reaction mixture contained approximately 10 ng of the extracted DNA, 1× PCR buffer (Promega, Madison, WI), 2.5 mM MgSO4, a mixture of 0.2 mM deoxynucleoside triphosphates (dNTPs), 0.5 U of GoTaq Flexi DNA polymerase (Promega, Madison, WI), and the forward and the reverse primers (10 μM [each]). A similar PCR protocol was used for all reactions, except for the change in the annealing temperature utilized. The PCR protocol was as follows: initial denaturation at 95°C for 5 min, followed by 40 cycles of denaturation at 95°C for 45 s, annealing at 46 to 54°C for 45 s, and elongation at 72°C for 90 s. A final elongation step at 72°C for 20 min was included. The highest possible annealing temperatures for the reverse primers were 54°C, 52°C, and 50°C for 1391R, 1492R, and 1525R, respectively. A trial-and-error approach was used to obtain a single-band PCR product with the expected size. For each lineage, the theoretical possible range of annealing temperatures was calculated as 2 to 10 degrees below the primer melting temperature (Tm −2 to Tm −10). For those forward primers that can theoretically anneal at temperatures above 54°C (the highest possible annealing temperature of all the reverse primers), the annealing temperature range was truncated to 54°C to Tm −10. PCR was first conducted at the highest possible annealing temperature using one reverse primer at a time. Temperatures were decreased sequentially, and the reverse primer was changed until a single-band PCR product was obtained at the correct size. In some instances, PCR products were not obtained despite our best efforts; i.e., after all possible forward-reverse primer pair combinations were tried at all possible annealing temperatures, no PCR product, a multiple-band PCR product, or a single-band PCR product of the incorrect size was obtained. For the rest of the lineages, a single-band PCR product of the correct size was obtained. The obtained PCR products were cloned using a TOPO TA cloning kit (Invitrogen Corp., Carlsbad, CA) according to the manufacturer's instructions. For each lineage, 8 clones were sequenced at the Oklahoma State core facility using both M13f and M13r primers to obtain a nearly full-length sequence. Forward and reverse sequences were assembled using DNAbaser V2 sequence assembly software (HeracleBioSoft, Pitesti, Romania). Chimeras were first checked using the command chimera.slayer (30) within MOTHUR (65). After removing potential chimeras, a second round of chimera checking was conducted on the remaining sequences using the Bellerophon program, available on the Greengenes website (14). The two-round chimera checking was essential to ensure that sequences were not considered novel based on a chimeric origin.

Phylogenetic analysis.

To determine whether the Sanger-generated sequences of a certain lineage were representative of the targeted short pyrosequencing-generated sequences, both the long and short sequences from each lineage were aligned using ClustalX (48), followed by trimming both the 5′ end (base pairs between 8f and the forward lineage-specific primer in the pyrosequencing reads) and the 3′ end (base pairs in Sanger-generated sequences past the 3′ end of the short sequences) in Jalview (73). The aligned truncated sequences were then analyzed using MOTHUR (65) to create a distance matrix followed by clustering them into OTUs. We considered a certain lineage to be successfully targeted when the Sanger-generated sequences and the short pyrosequences belonging to that lineage clustered in an operational taxonomic unit, with percent sequence divergence less than or equal to 15%. This rather stringent percent sequence divergence cutoff (15%, equivalent to a putative class level) was chosen as the criterion for defining target sequences to avoid any spurious effect a higher cutoff might have on falsely identifying target sequences.
Preliminary evaluation of phylogenetic affiliations of sequences obtained was conducted as follows: (i) comparison to the GenBank nr database using BlastN (42); (ii) alignment to the global 7,682-character Greengenes NAST-aligner (14) and subsequent classification by the use of the Greengenes taxonomic framework; and (iii) alignment to the global 7,682-character Greengenes NAST-aligner, import of these alignments into the Greengenes October 2010 database in the ARB software package (version 5.1-private-6215 M) (52), and determination of their positions after parsimony insertion into the universal ARB phylogenetic tree.
The combined output of such methods was used to identify lineages putatively representing novel phylogenetic lineages at the phylum or class levels. For detailed phylogenetic assessments, target sequences and their closest relatives (if any) were aligned to a collection of reference sequences representing 17 phyla and to 8 candidate phyla using ClustalX (48). The phylogenetic position of target sequences was evaluated using distance, parsimony, maximum-likelihood, and Bayesian approaches. Parsimony and distance neighbor-joining analyses were conducted using PAUP 4.0b (Sinauer Associates, Sunderland, MA) with the appropriate distance substitution model determined using ModelTest 3.7 (58). Maximum-likelihood analysis was conducted using RAxML 7.0 (68), and Bayesian analysis was conducted using MrBAyes 3.1 (37). Sequences were deemed representative of a new phylum if two or more distinct sequences remained reproducibly monophyletic and formed a bootstrap-supported independent cluster by the use of various character inputs, i.e., with or without a Lane mask, upon applying various tree-building algorithms as well as upon varying the composition and size of the data set used for phylogenetic analysis (12). The exact phylogenetic affiliation of novel lineages at the subphylum level was evaluated with special insertion into detailed phylum trees to determine the exact phylogenetic novelty of the lineage within this phylum.

Estimation of 16S rRNA gene copy numbers of proposed novel bacterial phyla by the use of qPCR.

The total copies of bacterial 16S rRNA genes per milligram of template DNA were quantified using universal primer pair 27F (5′-AGAGTTTGATCCTGGCTCAG-3′) and 338R (5′-GCTGCCTCCCGTAGGAGT-3′) (80). To quantify members belonging to the novel phyla proposed in this study, we used quantitative PCR (qPCR) with a forward primer specific to each phylum and the universal reverse primer 338R. Two of the proposed novel phyla (ZDP2 and ZDP5) were composed of a single pyrosequence lineage each, and all corresponding Sanger sequencing products obtained were representative of such lineages. Therefore, for ZDP2 and ZDP5, the same forward group-specific primer used for amplification of longer reads for Sanger sequencing (see above and Table 1) was utilized for and paired with the universal reverse primer 338R in the quantitative PCR analysis of the respective novel phylum. On the other hand, two of the proposed novel phyla (ZDP1 and ZDP4) were composed of 2 lineages each. One novel phylum (ZDP3) was composed of a single lineage, but that lineage also harbored sequences shown to belong to 2 distinct novel classes as well. Accordingly, for these 3 novel phyla (ZDP1, ZDP3, and ZDP4), designing separate qPCR phylum-specific forward primers was essential. The PRIMROSE program (3) was used to design such primers. The specificity was checked using RDP (11) against the probebase database and BLAST (42) against the nr database. The designed qPCR primers and the amplicon size expected when paired with the 338R universal reverse primer are shown in Table S2 in the supplemental material.
Table 1
Table 1 Novel phyla identified in this studya
Novel phylumPyrosequencing lineageSanger-generated sequence resultbqPCR quantification
No.eForward primercNo. obtained% similarity to closest GenBank relativeNo. of DNA copies/mg% of total 16S rRNA copies
PositionSequence
ZDP119188GGCTGACTGATAAAAGGG1819.6 × 104 ± 4.2 × 1030.5 ± 0.03
20177AATAGCATTGGAGAGTCG680–82
ZDP224d169TAATCCCGCATGTGCTCT586–884.9 × 103 ± 9 × 1020.026 ± 0.002
ZDP336d35TTGGCGGTGCGTCTTAGA281–825.2 × 102 ± 1 × 1020.002 ± 0.001
ZDP452d216CGGTCGCCATCGGATGAG483–841.2 × 105 ± 1.5 × 1030.62 ± 0.06
53d191CTTCCATTGATGAAAGGC783–84
ZDP565d63GTACGGAACTATGCTAGC277–847.7 × 104±3.4 × 1030.4 ± 0.06
a
Data represent group-specific forward primer sequences, percent similarity to GenBank first hits for the Sanger-generated sequences, and results of qPCR quantification for members of the proposed phyla.
b
Data represent Sanger-generated sequences obtained. No., number of sequences obtained for each lineage; % similarity, range of % sequence similarities to GenBank first hits for the sequences affiliated with each group.
c
Forward primer used in the PCR. Position corresponds to numbering of the E. coli 16S rRNA gene.
d
Primer may not be specific. Some of the Sanger-generated sequences in this group did not belong to that novel phylum.
e
Pyrosequencing lineage numbers refer to the number given to each of the 78 pyrosequencing lineages investigated for novelty in this study as described in detail in Table S1 in the supplemental material.
qPCR was conducted using a MyIQ thermocycler (Bio-Rad Laboratories, Hercules, CA) and B-R SYBR green Supermix for IQ (Quanta Biosciences Inc., Gaithersburg, MD). pCR4-TOPO (Life Technologies, Grand Island, NY) plasmids carrying phylum-specific 16S rRNA gene inserts were used both as qPCR-positive controls and standards for the corresponding novel phylum. Plasmids were extracted from clones grown overnight on LB plus kanamycin using a PureLink Quick Plasmid Miniprep kit (Life Technologies, Grand Island, NY) according to the manufacturer's instructions. Quantification of plasmid DNA concentrations was carried out using a Quant-iT double-stranded DNA (dsDNA) assay kit and a Qubit fluorometer (Life Technologies, Grand Island, NY). To avoid nonspecific target amplification, we conducted PCR at a range of annealing temperatures corresponding to 2 to 10°C below the theoretical primer melting temperature prior to the qPCR analysis. The highest annealing temperature resulting in a single-band PCR product of the correct size was chosen for downstream qPCR analysis. The specificity of primer pairs designed for qPCR (i.e., the ZDP1, ZDP3, and ZDP4 phylum-specific forward primers and the 338R universal reverse primer) was examined by cloning PCR products and subjecting 8 clones to Sanger sequencing. All clones were affiliated with the corresponding novel phylum (data not shown). qPCR was conducted using a 25-μl reaction mixture containing the forward phylum-specific primer (0.3 μM [each]) and 338R, 4.25 ng of template DNA, and 12.5 μl of B-R SYBR green Supermix for IQ. Since the expected amplicons ranged in size between 134 to 320 bp, a 3-step qPCR protocol was used as follows. Reactions were heated at 95°C for 3 min followed by 50 cycles of 20 s at 95°C, 45 s at the annealing temperature, and 45 s at 72°C.

Prediction of novel phylum 16S rRNA molecule secondary structure.

Small-subunit rRNA molecules are known to fold into conserved secondary and tertiary structures to facilitate interaction with ribosomal proteins (57). Early work by Woese and colleagues established the secondary structure of 16S rRNA (29, 56, 77) that was subsequently used as a model for its three-dimensional (3-D) folding (69). To confirm that the sequences representing each of the proposed novel phyla could be folded into credible secondary structures, a sequence representative of each novel phylum was first aligned with the Escherichia coli strain K-12 subsp. MDS42 16S rRNA gene sequence (GenBank accession number AP012306) by the use of ClustalX (48). Alignment was essential before attempting secondary-structure prediction to account for any insertions or deletions in the sequences. Alignments were visualized using Jalview (73). Since the bases corresponding to 1:X (where X denotes the position of the specific forward primer according to E. coli numbering) in sequences belonging to novel phyla were truncated, pairing of those bases with each other (e.g., bases 9 to 13 with bases 21 to 25 and bases 61 to 82 with bases 87 to 106) or with other bases (e.g., bases 17 to 19 with bases 916 to 918, bases 27 to 37 with bases 547 to 556, bases 39 to 46 with bases 395 to 403, bases 52 to 58 with bases 354 to 359, bases 113 to 115 with bases 312 to 314, and bases 122 to 142 with bases 221 to 239) could not be predicted in sequences belonging to most of the novel phyla. As a result, prediction of the secondary structure of the whole 16S rRNA molecule was not possible. To overcome this problem, guided by the alignment to the E. coli 16S rRNA gene sequence, we divided the representative 16S rRNA sequence belonging to each novel phylum into subsequences corresponding to E. coli base numbers 61 to 350 (only for the 2 novel phyla ZDP3 and ZDP5), 179 to 220 (only for the 2 novel phyla ZDP1 and ZDP2), 240 to 311 and 316 to 350 (only for the 3 novel phyla ZDP1, ZDP2, and ZDP4), and 367 to 393, 404 to 546, 556 to 915, and 920 to 1502 (for all novel phyla). Each of these subsequences was then submitted for secondary-structure prediction using the Mfold web server (82), followed by comparison of the minimum energy structure predicted to the conserved secondary structure of the E. coli 16S rRNA molecule.

Nucleotide sequence accession numbers.

The Sanger sequences obtained in this study have been deposited in GenBank under accession numbers JN387275 to JN387595.

RESULTS

Utility of a combined pyrosequencing-Sanger approach in obtaining representative sequences of target lineages.

Seventy-eight putatively novel lineages (1,992 sequence binned into 385 OTUs0.03) were determined based on automated classification of pyrosequences and subsequent insertion into an ARB global phylogenetic tree. Figure 1 represents an outline of the combined pyrosequencing-Sanger process. Details on the primers designed, reverse primer utilized, and annealing temperatures are presented in Table S1 in the supplemental material. Of the 78 lineages, 19 gave no clonable PCR products despite our best efforts, 7 gave a clonable product but no inserts were obtained upon sequencing, and 52 were successfully amplified and cloned and eight clones were sequenced per lineage. After checking and removing potential chimeras (8 sequences), 322 nearly full-length sequences were obtained and subjected to further analysis.
Fig 1
Fig 1 Flowchart of the combined pyrosequencing-Sanger approach for novel lineage identification.
Sanger sequences were compared to their respective pyrosequencing reads used for primer design to determine whether the Sanger sequence obtained represented the targeted pyrosequencing lineage according to the criteria outlined above. Of the 322 nearly full-length sequences analyzed, 124 sequences belonging to 32 lineages could confidently be labeled as target sequences, i.e., were less than 15% divergent from their target short pyrosequences. Within these 32 lineages, 17 were less than 5% divergent, 9 were more than 5% but less than 10% divergent, and 6 were more than 10% but less than 15% divergent from their target short pyrosequences. The large number of putative nontarget sequences is partly due to the fact that, while the designed primers were specific to the target pyrosequencing reads used for primer design and did contain mismatches to all publicly available sequences in ARB and RDP databases, 16S rRNA gene sequences that have no mismatches to the designed primer but do not belong to the target lineage are present within the extremely diverse source sediments in Zodletone Spring. Indeed, a significant fraction of nontarget sequences were not phylogenetically novel (see below) but belonged to well-described bacterial lineages. On the other hand, note that in some cases, confidently determining whether the obtained Sanger sequences belonged to target lineages was not feasible due to inadequate overlap between short and long sequences, since the designed primers were close to the 3′ end of the short pyrosequencing reads. Moreover, in some cases, Sanger-generated sequences were considered putative nontarget sequences due to a value of divergence from pyrosequencing-generated sequences only slightly higher than 15%, a threshold that could potentially be considered conservative, knowing the hypervariable nature of the overlap region.
Within the 32 lineages for which representative nearly full-length sequences were obtained, Sanger sequencing yielded exclusively target sequences in 13 lineages, while the nearly full-length sequences obtained for 19 lineages had both target and putative nontarget sequences (see Table S3 in the supplemental material).

Phylogenetic affiliations of the nearly full-length sequences obtained.

All nearly full-length sequences obtained were subjected to detailed phylogenetic analysis. Results identified multiple novel lineages at all high-rank taxonomic levels (phylum, class, order), as well as many representing novel families, genera, or species within identified lineages. Within the absolute majority of novel phyla and classes identified in this study, the nearly full-length sequences obtained were indeed representatives of the original pyrosequences used for primer design. In addition, in several cases, nearly full-length sequences representing two or more distinct pyrosequencing lineages clustered in a single, distinct, phylogenetically coherent monophyletic group. The fact that multiple short pyrosequencing lineages could cluster together to form one phylum is not surprising, since our initial identification of lineages in the pyrosequencing data set was based on a 15% sequence divergence cutoff and since the proportion of hypervariable bases in the V1-plus-V2 region is higher than that in nearly full-length fragments, thus inflating levels of sequence divergence in short fragments. Below, we present a detailed analysis of the phylogenetic affiliations of the nearly full-length sequences obtained. A list of all novel lineages encountered in this study is shown in Table S4 in the supplemental material.

Novel candidate phyla.

Detailed phylogenetic analysis identified five novel candidate phyla within the Zodletone Spring source community labeled ZDP1 to ZDP5 (Table 1). In total, 27 sequence representatives belonging to these 5 phyla were identified. The intralineage dissimilarity within each novel phylum ranged from 4% to 12%. These sequences originated from 7 different pyrosequencing lineages (Table 1). Sequences belonging to each of the novel phyla remained monophyletic and reproducibly formed a bootstrap-supported independent branch upon application of various tree-building algorithms (a distance neighbor-joining tree is shown in Fig. 2; parsimony, maximum-likelihood, and Bayesian trees are shown in Fig. S1 in the supplemental material).
Fig 2
Fig 2 Distance dendrogram based on the 16S rRNA Sanger-generated sequences affiliated with novel phyla (ZDP1 to ZDP5) encountered in the Zodletone Spring source sediment clone libraries. The tree was obtained using a Tamura-Nei substitution model with a proportion of invariable sites = 0.0606 and a variable site gamma distribution shape parameter = 0.8004. Bootstrap values (in percentages) are based on 1,000 replicates and are shown for branches with more than 50% bootstrap support.
Of the 27 sequences representing those novel phyla, only 3 were putative nontarget sequences (had more than 15% divergence from the corresponding short pyrosequences). In general, sequences belonging to these lineages have low sequence similarity to their closest relatives in public databases. Further, the closest relatives (e.g., the first five BLAST hits) of such sequences often belonged to multiple phylogenetic lineages, further suggesting that these novel sequences are not affiliated with any currently recognized phyla. Owing to the low percent similarity of sequences belonging to the proposed novel phyla to database-deposited 16S rRNA gene sequences, it was essential to ensure that those sequences were indeed representing small-subunit rRNA. We used Mfold to predict whether sequences belonging to the proposed novel phyla could fold into a credible small-subunit rRNA secondary structure. Characteristic secondary-structure features predicted for the proposed 5 novel phyla compared to those of E. coli are shown in Table S5 in the supplemental material. In general, all sequences representing novel phyla had the minimal features of small-subunit rRNA as described in reference 77. Variations from the conserved E. coli secondary structure were mainly in bases 144 to 219 (a multiple loop with 3 helices-hairpin loops corresponding to helices H8, H9, and H10 [76]) and 437 to 497 (a helix-internal loop-hairpin loop combination corresponding to helix H17 [76]) due to insertion or deletion in the corresponding bases in sequences belonging to the novel phyla. These helices were shown to interact with ribosomal proteins S4, S5, S12, S16, S17, and S20 (76). The base insertions or deletions led to a slightly shorter or slightly longer helix, internal loop, or hairpin loop (see Table S5 in the supplemental material). The only exceptions were sequences belonging to novel phylum ZDP2, where helix H10 was absent (see Table S5 in the supplemental material). However, according to reference 76, helix H10 does not seem to interact with any small subunit ribosomal protein, and so its presence might be dispensable.
Sequences belonging to novel phylum ZDP1 were most closely similar (80% to 82%, depending on the sequence compared) to those of uncultured microorganisms encountered in multiple habitats. Examples of the closest relatives were clones from the rhizosphere of phragmites at Sosei River in Sapporo, Japan (GenBank accession numbers AB240249, AB240377, and AB240493), clones from rice paddy soil (GenBank accession numbers AB486974 and AB487197) (41), and clones from microbial mats from Lower Kane Cave (Wyoming) (20) (GenBank accession number AM490643).
Sequences belonging to novel phylum ZDP2 were most closely similar (86% to 88%, depending on the sequence compared) to those of two clones from hydrothermally active sediments of the Guaymas Basin (GenBank accession number AF419661) (70) and from Lake Waiau sediment from the Hawaiian Archipelago (GenBank accession number AY345499) (16). Both clones do not belong to any of the recognized phyla within the Greengenes taxonomic schemes. Subsequent closest GenBank relatives were only 79% to 82% similar to ZDP2 sequences and were encountered in multiple habitats, e.g., an undisturbed tall grass prairie preserve in Kessler Farm Field Laboratory Biological Research Station in central Oklahoma (clones with GenBank accession numbers EU135313, EU135307, EU134920, EU134918, EU134910, and EU134671) (19) and a mud volcano sediment layer in the Neapolitan region of the eastern Mediterranean (GenBank accession number AY592608; see reference 34).
Sequences belonging to novel phylum ZDP3 were most closely similar (76% to 82%) to uncultured microorganisms encountered in multiple habitats. Examples include a clone from three microbial mat samples collected from volcano 1 on the Tonga-Kermadec arc (GenBank accession number HQ153894) (S. Murdock, H. Johnson, N. Forget, and K. S. Juniper, presented at the 4th International Symposium on Chemosynthesis-Based Ecosystems—Hydrothermal Vents, Seeps and Other Reducing Habitats, Roscoff, France, 2010), sequences from subseafloor sediment of the South China sea (GenBank accession number EU385810), and a clone with GenBank accession number AM490651 from the microbial mats in the Lower Kane Cave (20).
Sequences belonging to novel phylum ZDP4 were most closely similar (81% to 84%, depending on the sequence compared) to those of uncultured microorganisms from multiple habitats. The first 100 GenBank hits were all within the same range of percent similarity to ZDP4 sequences (79% to 84%) and corresponded to clones from 10 different studies. Examples include a clone from a biogas fermentation enrichment culture (GenBank accession number GU476604), a clone from a mesophilic biogas digester treating pig manure (GenBank accession number EU358689) (50), and a clone from water samples of Lake Cadagno, an alpine meromictic lake located in the Piora valley in the Southern Alps of Switzerland (GenBank accession number FJ502268) (31).
Finally, sequences belonging to novel phylum ZDP5 were most closely similar (77% to 84%) to a number of sequences retrieved from a few environmental surveys. These include clones from a mesophilic anaerobic reactor fed with effluent from the chemical industry in Mexico City, Mexico (GenBank accession numbers FJ462088, FJ462086, FJ462085, and FJ462087) and a clone from Fe-Mn nodules and surrounding soil in Wuhan, Hubei Province, Central China (GenBank accession number EF492917) (33).
As a further proof of their presence in Zodletone Spring sediments, we quantified members belonging to the above 5 proposed novel phyla by the use of qPCR. Results of qPCR quantification are shown in Table 1. Members belonging to novel phyla ZDP1 to ZDP5 represent 0.002% to 0.6% of the 16S rRNA genes identified by qPCR using universal primers.

Novel classes.

In addition to the novel phyla identified, multiple targeted lineages formed unique monophyletic clusters within existing phyla and candidate phyla. Ten novel classes are proposed from 13 different pyrosequencing lineages (Table 2). Of the 55 sequences representing those novel classes, 17 were putative nontarget sequences with >15% sequence divergence from their corresponding short sequences. The intralineage diversity within each novel class ranged from 4% to 9%. A distance neighbor-joining tree of novel classes is presented in Fig. 3 (parsimony, maximum-likelihood, and Bayesian trees are shown in Fig. S2 in the supplemental material). As shown, novel classes in 2 previously recognized phyla (Planctomycetes and Fibrobacteres) and 7 previously recognized candidate phyla (BRC1, GN06, GN12, LD1, TM6, WS2, and TM7) were identified.
Table 2
Table 2 Group name, primer sequence, and percent similarity and accession numbers of GenBank first hits for the Sanger-generated sequences belonging to putative novel classes identified in this study
Novel class namePhylumaPyrosequencing lineageSanger-generated sequence resultb
No.eForward primercNo. obtainedClosest relative in GenBank
PositionSequence% similarityGenBank accession no.
ZDC1Planctomycetes4d198GGATTTTCGGACCTTCTG289BX294875
77d177GATGTGACCACACTGGCG179EU287119
ZDC2Fibrobacteres54176GGATATTGTGGAGCATCG882–83FJ716839
ZDC3BRC136d35TTGGCGGTGCGTCTTAGA378–80GU172181
ZDC4GN1249190CGGCGATGAGCAAAGATG480–83GQ472374, EU048619
ZDC5TM658173ACAGCATACGTCTTTTCG382–84GQ246408, FJ264771
ZDC6LD167d125GGGTACTTGCCCTCGACT778–80HQ174951
ZDC7TM775173ACTCCATGTGGTCTTACG881–85FJ542972
ZDC8GN06766TGCGAGGCGGTTCTTCGG486–87GQ246356, FJ264777
ZDC9GN069d55TGCAAGTCGGATGCGAAA389–90EU245453
10176GCATACGCTTGTCTCTGT787–92EU245453, FJ516883, EU245159
ZDC10WS233d162GCGCCGCTAATACCGGGT490–91EU385957, DQ787710
36d35TTGGCGGTGCGTCTTAGA190EU385957
a
Phylum to which the putative novel class in column 1 belongs.
b
Sanger-generated sequences obtained. No., number of sequences obtained for each lineage, % similarity, range of percent sequence similarities to GenBank first hits for the sequences affiliated with each group. GenBank bank accession numbers of those first hits are shown.
c
Forward primer used in the PCR. Position corresponds to numbering of the E. coli 16S rRNA gene.
d
Primer may not be specific. Some of the Sanger-generated sequences in this group did not belong to that novel class.
e
Pyrosequencing lineage numbers refer to the number given to each of the 78 pyrosequencing lineages investigated for novelty in this study as described in detail in Table S1 in the supplemental material.
Fig 3
Fig 3 Distance dendrogram based on the 16S rRNA Sanger-generated sequences affiliated with novel classes (ZDC1 to ZDC10) encountered in Zodletone Spring source sediment clone libraries. The tree was obtained using a Tamura-Nei substitution model with a proportion of invariable sites = 0.0908 and a variable site gamma distribution shape parameter = 0.7571. Bootstrap values (in percentages) are based on 1,000 replicates and are shown for branches with more than 50% bootstrap support.
Planctomycetes class ZDC1 formed a monophyletic lineage independent of the 10 recognized classes within the phylum according to the Greengenes taxonomy (classes Kuenenia, Phycisphaerae, Planctomycea, C6, FFCH393, Koll-18, ODP123, PW285, agg27, and VadinHA49). Fibrobacteres class ZDC2 formed a monophyletic cluster independent of the 2 recognized classes within the phylum Fibrobacteres (Fibrobacteres and Fibrobacteres 2). Similar patterns of forming novel class-level independent lineages with low sequence similarity to closest relatives were observed with classes ZDC3, ZDC4, ZDC5, ZDC6, ZDC7, ZDC8, ZDC9, and ZDC10 within the candidate phyla BRC1, GN12, TM6, LD1, TM7, GN06 (for two classes), and WS2 respectively.

Novel orders.

We identified 19 novel orders belonging to 21 lineages within previously described classes in 4 phyla (Planctomycetes, Proteobacteria, Chloroflexi, and Verrucomicrobia) and 6 candidate phyla (GN06, OP3, OP8, OP11, TG3, and WS6) according to the criteria described above. Details are shown in Table 3. Nine of 21 lineages were target sequences. Novel Planctomycetes orders were within classes Phycisphaerae (ZDO1, ZDO2) and VadinHA49 (ZDO3, ZDO4); novel Chloroflexi orders were within classes Chloroflexi (ZDO5) and Anaerolineae (ZDO6); novel Proteobacteria orders were within class Deltaproteobacteria (ZDO7, ZDO8, ZDO9); and novel Verrucomicrobia orders were within classes R76 to B18 (ZDO10). Similarly, patterns of forming novel order-level independent lineages with low sequence similarity to closest relatives were observed with orders ZDO11 to ZDO19 within previously described classes of candidate phyla TG3, WS6, OP8, OP3 (3 novel orders), OP11 (2 novel orders), and GNO6, respectively.
Table 3
Table 3 Group name, primer sequence, and percent similarity and accession numbers of GenBank first hits for the Sanger-generated sequences belonging to putative novel orders identified in this study
Novel order namePhylumaClassbPyrosequencing lineageSanger-generated sequencesc
No.fForward primerdNo.Closest relative in GenBank
PositionSequence% similarityGenBank accession no.
ZDO1ChloroflexiChloroflexi39e183CCACAGAGTCTTCGGGCT291AB473922
ZDO2Anaerolineae13e120CACGTGGGTCATTTGCCC186DQ811881
ZDO3ProteobacteriaDelta33e162GCGCCGCTAATACCGGGT284–85AB630482
ZDO4Delta56e36GGGCCGGGCGTGCTTAAC181HQ397064
ZDO5Delta67e125GGGTACTTGCCCTCGACT193AY499723
ZDO6VerrucomicrobiaR76–B1827274GGCTTACGGGTAGTTGGT591–93EU287160, FJ416080
ZDO7PlanctomycetesPhycisphaerae3129AACGTACCCATAGCACGG583–88FJ710716
ZDO8Phycisphaerae61e154GTCCCGAAAGGGGCGGTA390–93HQ405605, HQ183982, EF125448
ZDO9VadinHA494e198GGATTTTCGGACCTTCTG485–89AB231079, AB522154, CU919930, FJ374260
ZDO10VadinHA495197GGTTGGGATCTATGGACC784–86AF149887, AB234542
ZDO11TG3TG3–142e230AGCTTGCGGGCCCATTAG386–90GU476604
44e183GATCGGCTTGGCGCATGT387–89GQ458248
4566CGTGAAGCTCAGGCAACT788GQ458248, GU476604
ZDO12WS6SC726e230GGATTGCGTCCTATCAGC184GU363049
ZDO13OP8OP8–263209ATTAGGATCAAAGGGGGG287EF688191
ZDO14OP3BD4–96e230GGATTGCGTCCTATCAGC189AY792312
ZDO15PBS–2513e120CACGTGGGTCATTTGCCC190AJ390463
ZDO16Koll–116e230GGATTGCGTCCTATCAGC186AM991240
ZDO17OP11OP11–278e85GTATCGCGCTCTTAGCGG385–87AY218580, AF419686
ZDO18WCHB1–646e230GGATTGCGTCCTATCAGC180AB510995
ZDO19GN06KSB360e232CCCGCGGACTATTAGTTA189FM242438
a
Phylum to which the putative novel order in column 1 belongs.
b
Class to which the putative novel order in column 1 belongs.
c
Sanger-generated sequences obtained. No., number of sequences obtained for each lineage, % similarity, range of percent sequence similarities to GenBank first hits for the sequences affiliated with each group. GenBank bank accession numbers of those first hits are shown.
d
Forward primer used in the PCR. Position corresponds to numbering of the E. coli 16S rRNA gene.
e
Primer may not be specific. Some of the Sanger-generated sequences in this group did not belong to that novel class.
f
Pyrosequencing lineage numbers refer to the number given to each of the 78 pyrosequencing lineages investigated for novelty in this study as described in detail in Table S1, in the supplemental material.

Other sequences.

The remaining sequences (n = 154) belonged to 19 distinct lineages (8 of the 19 lineages were target sequences) and formed 18 distinct families, 14 distinct genera, and 20 distinct species. Details are shown in Table S6 in the supplemental material.

Correlation between value of sequence similarity to closest relative of a pyrosequencing read and its “true” phylogenetic affiliation.

Based on the results obtained, we sought to determine whether the percent similarity to closest relatives obtained for a specific pyrosequencing-generated read could accurately predict its putative phylogenetic novelty, i.e., whether such sequences truly represent a novel phylogenetic lineage when a nearly full-length sequence is obtained and analyzed. Such quantification could potentially contribute to better judgment in identification of putatively novel lineages within pyrosequencing data sets in similar future studies.
For each of the 32 lineages whose Sanger-generated sequences were indeed representative of their corresponding pyrosequencing reads, the original pyrosequencing reads used for the forward primer design were identified. These reads (1,566 sequences, 192 OTUs) were compared to the NCBI nr database using BLAST nr (42), and the percent sequence divergence from the nearest relative was determined. Within those lineages, 10 were proposed as members of novel phyla, 6 were proposed as members of novel classes, 9 were proposed as members of novel orders, and 6 were proposed as members of novel families, genera, or species. The correlation between the percent sequence similarity to closest relative and “true” phylogenetic affiliation obtained from examining the corresponding Sanger sequence was analyzed. Results (Table 4) indicate averages of 78.8%, 81.3%, and 86.3% for phyla, classes, and orders, respectively, with a low coefficient of variation (0% to 4.29%). Therefore, for an 8f-338R pyrosequencing-generated 16S rRNA data set, sequences with values around these thresholds should be used in similar future protocols to maximize the potential for identifying novel phyla, classes, and orders.
Table 4
Table 4 Correlation between sequence similarity to closest relative value of a pyrosequencing read and its true phylogenetic affiliation
Taxonomic levelaLineage (group)bAverage % similarity to closest database relative (CV)cAverage % similarity for that taxonomic leveld
Phylum1981 (0)78.8
2076 (one sequence)
2482.5 (4.29)
5278.9 (1.71)
5377.5 (1.08)
6577 (0)
Class481 (one sequence)81.3
784 (1.77)
980 (3.51)
1080 (0)
3380 (1.15)
3680.5 (0.55)
4986 (0)
5479 (one sequence)
5879 (0.65)
7783 (0)
Order385 (0.51)86.3
688.7 (3.51)
1380.4 (0.98)
2785 (one sequence)
3586 (one sequence)
4287.3 (one sequence)
4487.8 (one sequence)
4589.3 (one sequence)
6387.5 (3.4)
Family (or lower)2385 (0)88.3
5187 (one sequence)
5990 (one sequence)
6089 (one sequence)
6290.5 (0.88)
7683.9 (1.11)
7893 (0.88)
a
Phylogenetic affiliation of only target Sanger-generated sequences belonging to lineages shown in column 2.
b
Numbers refer to the pyrosequencing lineage numbers given to the 32 target lineages used for this correlation.
c
Average percentage sequence similarity to the nearest GenBank relative for all pyrosequences belonging to the lineages shown in column 2. For each of the lineages shown, all pyrosequences were identified and compared to NCBI nr database using BLAST, and the average percent sequence similarity to the first hit (closest relative) was calculated. Numbers in parentheses represent the coefficients of variation of percent sequence similarities to closest relatives of all pyrosequences belonging to that lineage calculated as CV = SDmean × 100, where CV is the coefficient of variation, SD is the standard deviation, and mean is average percent similarity.
d
For each taxonomic level, the number corresponds to the average of all percent similarities shown in column 3 for that taxonomic level.

DISCUSSION

In this study, a direct, straightforward, and fairly cost-effective approach to assessment of the phylogenetic affiliations of multiple unclassified reads generated during 16S rRNA gene pyrosequencing surveys of specific ecosystems is presented. The combined Sanger-pyrosequencing approach proposed in this study produces high-quality sequences with read lengths of 1,100 to 1,500 bp, values that exceed the newest GS-FLX+ Titanium chemistry pyrosequencing read lengths (average read length, 700 bp [September 2011]). However, unlike the sequences with shorter read lengths generated by next-generation sequencing technologies, the Sanger-generated sequences would be available in curated databases for future reference and comparison, and sequencing error rates for Sanger-generated sequences (22, 35) are considerably lower than the sequencing error rates generated by pyrosequencing (35). Using this combined approach, we show that it is possible to retrieve nearly full-length sequences of targeted pyrosequencing lineages by pairing a lineage-specific primer to a universal reverse primer. We also show that, while the majority of examined pyrosequencing reads that are unclassified within our data set could be accurately assigned to known classes, orders, and families, a fraction of the unclassified sequences indeed represents previously unencountered novel phyla (ZDP1 to ZDP5) and novel classes (ZDC1 to ZDC10) within the domain Bacteria.
The rise and currently nearly exclusive utilization of high-throughput sequencing technologies is understandable due to the cost advantage and versatility compared to Sanger sequencing. Use of those technologies has resulted in valuable ecological insights, e.g., the more accurate estimates of species richness and evenness (59, 60, 63, 81) and the more detailed comparisons between communities at various temporal (17, 44, 46) and spatial (6, 17, 24, 49, 75) gradients. Unfortunately, the rise of such sequencing technologies has resulted in overlooking detailed phylogenetic analysis due to the short amplicon size and the sheer number of clone sequences analyzed. The approach outlined in this study thus offers a valid alternative for exploring the phylogenetic diversity of putatively novel members of the rare biosphere. To our knowledge, only one previous study has utilized a similar approach, on a much narrower scale, to explore the phylogenetic affiliation of selected marine sponge-affiliated tag sequences with <75% similarity to 16S rRNA sequences in the curated SILVA database (74). However, the study yielded only 4 Sanger sequences and provided no detailed methodological information regarding primer design, success rates in obtaining nearly full-length sequences representative of targeted lineages, and proportions of novel high-rank diversity (e.g., candidate phyla, classes, and orders) obtained as a fraction of the total number of lineages examined. Further, the study paired a 616F general bacterial primer with V6 lineage-specific reverse primers, resulting in relatively short sequences (approximately 900 bp) that would not be deposited and classified in curated databases, e.g., Greengenes and SILVA. The current report, on the other hand, represents an ecosystem-wide evaluation of the phylogenetic affiliation of a large number of unclassified lineages from a pyrosequencing data set. The report also provides detailed information on (i) criteria for the selection of pyrosequencing lineages for this type of analysis; (ii) the utility and value of utilizing various universal reverse primers in obtaining nearly full-length sequences of targeted lineages; (iii) criteria for lineage-specific primer design and evaluation; (iv) details on the success rate of such an approach; (v) evidence demonstrating the importance of sequencing multiple clones per targeted lineage; (vi) a detailed methodology regarding the detection and quantification of nontarget versus target amplification of nearly full-length 16S rRNA gene sequences; and (vii) detailed information regarding the correlation between the value of sequence similarity to the closest relative of a pyrosequencing read and its “true” phylogenetic affiliation.
The identification of five novel candidate phyla in a single study is a significant expansion of the scope of diversity within the domain bacteria. Currently, 84 phyla and candidate phyla have been described (according to the latest update of Greengenes taxonomy [July 2011]) [14]). While earlier diversity surveys (see, e.g., references 15 and 38) had often been successful in identifying multiple novel phyla within a single study using a relatively small number of sequences, the pace of discovery of novel phyla has subsequently slowed due to the wide utilization of 16S rRNA surveys in examining diversity in almost all accessible habitats on earth. The apparent saturation of phylum-level diversity, however, was challenged by the discovery of novel, putatively distinct lineages of rare members of microbial communities in recent high-throughput diversity surveys. This report demonstrates that a fraction of the unclassifiable members of the spring community that have not been accessed in Sanger surveys due to low abundance indeed belongs to novel microbial phyla and that a targeted approach that combines the high-throughput capabilities of pyrosequencing and the read length of Sanger sequencing can identify and accurately describe such lineages. Therefore, we reason that a similar approach, conducted on multiple ecosystems, and at more depth (more lineages per sample), could indeed greatly expand the scope of understanding of bacterial (and putatively archaeal) phylogenetic diversity on earth. The reason for the presence and maintenance of these lineages at low abundance, as well as their global distribution patterns and putative ecological roles (or lack thereof), would be an area of interest to microbial ecologists and evolutionary microbiologists.

ACKNOWLEDGMENT

This work was supported by the National Science Foundation Microbial Observatories Program (grant EF0801858).

Supplemental Material

File (aem2-12_figs1-s2.pdf)
File (aem2-12_tabless1-s4.pdf)
File (aem2-12_tabless5-s6.pdf)
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Applied and Environmental Microbiology cover image
Applied and Environmental Microbiology
Volume 78Number 815 April 2012
Pages: 2677 - 2688
PubMed: 22307312

History

Received: 3 January 2012
Accepted: 27 January 2012
Published online: 23 March 2012

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Noha Youssef
Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma, USA
Brandi L. Steidley
Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma, USA
Present address: Department of Microbiology, Ohio State University, Columbus, Ohio, USA.
Mostafa S. Elshahed
Department of Microbiology and Molecular Genetics, Oklahoma State University, Stillwater, Oklahoma, USA

Notes

Address correspondence to Mostafa S. Elshahed, [email protected].

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