INTRODUCTION
Urinary tract infections (UTIs) are among the most common bacterial infections in childhood. Pediatric UTI affects between 2.5% and 5% of children annually, accounting for over 1 million office visits, 500,000 emergency department visits, and 50,000 hospital admissions in the United States annually (
1–6). Importantly, pediatric patients are at risk for long-term sequelae following UTIs, including renal scarring and progressive renal disease, with lifelong deleterious effects on health (
7,
8).
Escherichia coli is the most common uropathogen in pediatric populations, accounting for 70–90% of first UTIs (
4,
6,
9). Translocation of uropathogenic
E. coli from the gastrointestinal (GI) tract of the affected individual via the fecal-perineal-urethral pathway is considered the dominant route of infection (
10–13). The
E. coli strains colonizing the GI can range from benign colonizers to extraintestinal pathogenic strains. The GI tract colonization process is shaped by a combination of host characteristics and environmental exposures (
14,
15) including close household contacts, companion animals, and food (
10,
12,
16,
17).
Retail poultry has been identified as a potential reservoir of
E. coli capable of causing extraintestinal infections in humans. In adults, poultry consumption is a risk factor for UTIs (
18,
19) and genetic analyses have demonstrated similarities between
E. coli contaminating retail meats and those causing human UTIs (
20–22). However, it is infeasible to sufficiently sample the roughly 9 billion poultry raised for meat annually in the United States to define the
E. coli populations colonizing these animals or to recognize poultry-to-human spillover infections using core-genome phylogenetic methods (
23).
E. coli strains appear to acquire and shed host-adaptive genes located on mobile genetic elements (MGEs) as they transition between vertebrate hosts (
23,
24). Previously, we developed a Bayesian latent class model (BLCM) to use the presence and absence of 17 source-associated MGEs to predict the origins of
E. coli isolates in a predominantly adult population in Flagstaff, Arizona (
23). In the current study, we used this model to quantify the burden of pediatric UTIs caused by the spillover of foodborne zoonotic
E. coli (FZEC) in Washington DC.
RESULTS
Clinical E. coli isolates were collected from 52 Washington DC residents aged 2 months to 17 years, including 12 infants (0–12 months), 16 toddlers (1–3 years), 11 young children (4–8 years), and 13 adolescents (10–17 years). Most patients were female (82.7%, n = 43/52). Fever was reported as the most common indication for urine culture among infants (n = 11/12) and was also common among toddlers (n = 6/16). Dysuria and other voiding-related symptoms, with or without abdominal/flank pain/hematuria, were the most common indications among toddlers (n = 10/16), young children (n = 7/11), and adolescents (n = 11/13).
To characterize E. coli populations circulating among retail poultry products in Washington DC, we sampled all available brands of poultry from 15 grocery stores located throughout all wards (i.e., administrative divisions). We purchased and processed 94 raw chicken and 45 raw turkey products. We recovered 106 E. coli isolates (67 chicken and 39 turkey isolates) from 56 unique poultry samples. One E. coli isolate per positive poultry sample (n = 56) was arbitrarily selected for sequencing.
We identified 56 multilocus sequence typing (MLST) sequence types (STs) among all the sequenced isolates (
Fig. 1 – phylogenetic tree;
Table S1). We found nearly two times as many STs from the poultry isolates than from urine isolates (40 vs 22 STs;
P < 0.0001). Five of these sequence types—ST10, ST38, ST69, ST117, and ST131—were identified among both poultry and human isolates. Twenty-three clinical urine isolates (44.2%,
n = 23/52) and 14 poultry isolates (25.0%,
n = 14/56) belonged to these shared sequence types.
The Bayesian latent class model predicted that 10 of the 52 clinical urine isolates (19%) were of meat origin (probability ≥0.8); whereas the model predicted that all 56 poultry
E. coli isolates were of meat origin (
Table S2). Six of the 10 clinical isolates predicted to be of meat origin were from the five shared STs, including ST10 (
n = 1), ST117 (
n = 3), ST38 (
n = 1), and ST69 (
n = 1). Our model predicted four other clinical
E. coli isolates to be of meat origin, despite belonging to sequence types not observed among the poultry isolates sampled here. These sequence types—ST101 (
n = 1), ST2556 (
n = 1), ST388 (
n = 1), and ST95 (
n = 1)—have been observed among meat isolates in previous studies (
23).
The patients infected by strains of putative meat origin were all female and represented each of the pediatric age groups, including infants (n = 2/12), toddlers (n = 1/16), young children (n = 4/11), and adolescents (n = 3/13).
We compared the resistance gene profiles of FZEC, non-FZEC, and meat isolates. FZEC isolates tended to carry fewer genes conferring resistance to medically important antibiotics as compared to non-FZEC or meat isolates (
Table S3). FZEC and meat isolates were numerically more likely to carry tetracycline-resistance genes as compared to non-FZEC clinical isolates, but this difference was not statistically significant.
DISCUSSION
Our analysis, using a novel Bayesian latent class model, suggests that over 19% of the pediatric UTIs in this study were caused by
E. coli strains with a high likelihood of originating from meat. This was more than twice the spillover rate of FZEC previously estimated for a population consisting primarily of American adults (
23). None of the poultry isolates were predicted to be of human origin.
The analytical approach that we used in this study is a powerful tool for recognizing zoonotic E. coli strains among extraintestinal infections in pediatric and adult populations. Core-genome phylogenetic analysis has become a cornerstone of outbreak investigations; however, it has limitations when it comes to recognizing sporadic host spillover events, especially for an organism as diverse as E. coli. This is further compounded by the enormity of food animal populations that may serve as reservoirs for zoonotic strains, which makes it practically impossible to sample the food supply sufficiently to define transmission patterns. By moving beyond the core genome to identify mobile genetic elements that are differentially associated with humans and the dominant food-animal species (or meat types), we have developed an approach that estimates the probability that an isolate originated from a particular source. The results for this Washington DC-based pediatric population represent the successful extension of the BLCM from our prior analysis to a geographically, demographically, and temporally distinct population. As we expand our panel of host-associated mobile genetic elements and reference genomes, we anticipate creating a tool that can be used by the infectious diseases community to quantify the burden of zoonotic E. coli strains to the burden of extraintestinal infections worldwide.
The study’s primary limitation was its small sample size, which made it difficult to make quantitative statements about the foodborne zoonotic
E. coli strains that pose the greatest risk to pediatric populations. Despite this limitation, one sequence type, ST117, was found to be the most common sequence type among putative zoonotic
E. coli infections in this study, consistent with previous work and was the most prevalent sequence type among
E. coli isolated from poultry products (
23). However, despite its prevalence in meat, ST117 was not the most common zoonotic lineage among extraintestinal infections in adults in the previous study. Future studies will have to be conducted to determine if ST117 poses a particular risk to pediatric populations and if these infections are due to specific virulence factors associated with this lineage, due to a simple stochastic effect of being so prevalent in meat, or a combination thereof. Additionally, our study included participants from a wide age range (i.e., from 2 months to 17 years), with substantially different pathophysiology, epidemiology, and etiology of UTIs. Therefore, future studies are also needed to further investigate the impact and risk factors of foodborne zoonotic
E. coli infection across the pediatric age span.
Our findings, and those of others (
18,
25–28), suggest that raw poultry is one vehicle for the transmission of uropathogenic
E. coli strains between food animals and humans. However, the linkage is not straightforward in that young children are unlikely to handle raw poultry. Thus, other incidental exposures, such as contaminated kitchen surfaces, hands of caregivers, cross-contaminated foods, undercooked poultry, and colonized household members, including pets, are all potential routes of transmission. Unfortunately, none of these potential intermediate sources were sampled for this study.
Our findings give further context to the potential risks associated with antimicrobial use in food-animal production. However, in this study we found that the putative FZEC clinical isolates tended to carry fewer antimicrobial resistance genes as compared to non-FZEC clinical isolates. This likely reflects the relative selective pressure that these populations have undergone. Over the past two decades, the U.S. Food and Drug Administration has established more protective guidelines for the introduction of new antimicrobials to food animal production and has restricted the use of some previously approved antimicrobials (
29). Despite this progress, tetracyclines are still routinely used in U.S. food animal production, which may explain the high prevalence of tetracycline resistance genes among the FZEC and meat isolates. Further investigations are needed to assess the variability in antimicrobial susceptibility among pediatric FZEC isolates in countries with different antimicrobial policies and practices (
30).
UTIs account for billions of dollars in healthcare-associated costs and can impart serious long-term sequelae, particularly among pediatric patients. Understanding the epidemiology of UTI could reveal critical control points in the pathway from environmental reservoirs to infection. Here, we demonstrate evidence that retail poultry meat can serve as a vehicle for extraintestinal pathogenic E. coli with the potential to cause pediatric UTI. We speculate that children are exposed to these pathogens indirectly and that transmission may be reduced by improving kitchen and hand hygiene, as well as industry measures—such as vaccination programs—to decrease uropathogenic E. coli populations in food animals. Future studies should be conducted to measure the rate of FZEC infections in different populations that vary in age, race, geography, socioeconomic status, and public health infrastructure (i.e., water, sanitation, and hygiene).
ACKNOWLEDGMENTS
This work was supported by Wellcome Trust (award #201866), National Institutes of Health [grant #5R21AI117654-02 (L.B.P.), #1R01AI130066-01A1 (L.B.P.)], and the George Washington University Food for Thought Pilot Award. The funding sources had no role in the study design, data collection, analysis, interpretation, or the writing of this manuscript.
M.A. performed formal analysis, visualization, project administration, and manuscript preparation; G.S.D. performed laboratory research, study coordination, and manuscript preparation; D.E.P. performed methodology development, visualization, formal analysis, and manuscript preparation; Y.W. and S.S. performed formal analysis and visualization; A.H.I. performed clinical data analysis; S.Z. performed sample collection; L.N. performed study coordination; B.W. and S.S. performed laboratory analysis; Z.W. and T.J.J. contributed to method development; J.C. performed clinical isolate collection; E.C.-N. and K.A.C. performed bioinformatic analysis; C.M.L. conceptualized the study and performed formal analysis, project administration, and manuscript preparation; R.L.D. designed the clinical study and performed data analysis and manuscript preparation; L.B.P. conceptualized the study and performed formal analysis, manuscript preparation, project administration, and funding acquisition.