Is Fecal Carriage of Extended-Spectrum-β-Lactamase-Producing Escherichia coli in Urban Rats a Risk for Public Health?

Brown rats (Rattus norvegicus) are synanthropic and inhabit urban infrastructures. Therefore, they might be involved in transmission pathways of zoonotic bacteria, including multiresistant “superbugs” like extended-spectrum beta-lactamase-producing (ESBL) Escherichia coli (1). According to our previous data, ESBL E. coli organisms are apparently present in the feces of urban brown rats from Europe (2, 3). As in these studies, however, nonselective media were used; they were not adequate to estimate an approximate rate of ESBL E. coli colonization of the gut of rats.

Thus, it is still necessary to determine the importance of the gut of urban rats as a reservoir for ESBL E. coli to verify if previous findings of ESBL E. coli might have been a coincidence, which would suggest a low relevance for the transmission of these bacteria by rats.

In 2010, we screened a total number of 56 brown rats for ESBL E. coli by plating fecal contents using selective CHROMagar (Mast Diagnostica, Reinfeld, Germany) supplemented with cefotaxime (4 μg/ml). The animals were obtained from 19 different sampling spots covering the inner-city area of Berlin (Germany). They were trapped during pest control procedures either in buildings and public areas like parks and streets (n = 47) or in sewer tunnels close to the wastewater discharge of a university hospital (n = 9). E. coli isolates with confirmed ESBL production according to a Clinical and Laboratory Standards Institute (CLSI) document (4) were further analyzed for (i) their phenotypic resistance to several antimicrobials by agar disc diffusion, (ii) their possession of antimicrobial resistance genes via PCR, and (iii) their phylogenetic background via multilocus sequence typing (MLST) and structure analysis. Their clonal relatedness was examined by means of pulsed-field gel electrophoresis (PFGE). All tests were performed using protocols described previously (3). Overall, 16% of the rats examined carried an ESBL E. coli strain. The detected rates not only were significantly higher than those reported for rats from China and Senegal (0.5% to 4%) (5–7) but also exceeded those that have been recently reported for healthy individuals from comparable urban settings (5% to 8%) (8–11). On the other hand, they were similar to the prevalence of ESBL E. coli in hospitalized patients or their household contacts (12% to 16%) (12). In particular, the high number of ESBL E. coli isolates determined among sewer rats, which were trapped near the wastewater discharge of a large hospital, might be an indication for high ESBL E. coli levels in the outflow and for a permanent transmission of these bacteria from clinical environments to the rat population. This may also explain the high diversity of ESBL-producing bacteria that have been found in a recent study in urban river sediments (13).

The most prevalent ESBL gene detected among the rat isolates was bla_CTX-M-1 (87.5%) (Fig. 1), which is common in human and livestock samples in Europe (14). All ESBL E. coli isolates harbored transferable large resistance plasmids of >100 kb belonging to inc/rep probe type FIA or FIB. Most of the isolates showed combined resistance to other antimicrobial classes, including fluoroquinolones, tetracyclines, and aminoglycosides (Fig. 1). Multilocus sequence types (STs) included ST10 (ST complex 10), ST410, and ST90 (both ST complex 23), and these are well-known STs frequently associated with an ESBL phenotype also in human and veterinary clinical isolates (14). ST90 ESBL E. coli strains representing a single clone could be traced via PFGE in three different animals over a period of 2 months (Fig. 1). This clone initially appeared in two animals captured in the same sampling spot in the sewage system within 2 weeks of each other. Six weeks later, it was recovered from a third animal in a nearby apartment (distance, 700 m), which the rat possibly entered through the toilet drain. This finding points toward a spread of ESBL E. coli from the sewage system to human infrastructures by rats, which might present an important vector within those cycles. The potential dissemination of different types of ESBL E. coli isolates by rats is further exemplified by one animal (no. 6) (Fig. 1) which carried two different strains. These varied in sequence type (ST10 versus ST34) and ESBL type (CTX-M-1 versus SHV-12). Furthermore, rats from the sewer tunnels carried ESBL E. coli isolates twice more often (33%) than did the total rat population sampled (16%). This may reflect a bias due to the small number of animals available from the sewage system (n = 9), which is a result of the legally restricted access to the sewage system in Berlin. We also observed that the rats tended to avoid the traps after one of their conspecifics had been captured. It must be conceded that the study is based on a limited number of animals. This is due to difficulties in rat sampling, which somehow reflects the obstacles leading to constricted pest control. Nevertheless, our results reveal that urban rats might be of importance with regard to public health, as they carry high rates of ESBL E. coli strains that have genotypes and ESBL types resembling those that currently circulate in human patients and thus have to be considered zoonotic. Urban areas are assumed to be populated by hundreds of thousands of rats (15, 16). Rat feces are therefore considered to be omnipresent and are most likely a permanent environmental source of zoonotic and multiresistant bacteria. There is an urgent need for holistic approaches comprising humans, animals, and the environment to explore putative transmission cycles of multiresistant ESBL E. coli strains in the future.

This work was supported by the Federal Ministry of Education and Research network Food-Borne Zoonotic Infections of Humans (FBI-Zoo grant 01KI1012A to L.H.W.) and NaÜPa-Net (Netzwerk Nagetierübertragene Pathogene; grant 01KI1018 to R.G.U.).