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Applied and Environmental Microbiology, January 2007, p. 477-484, Vol. 73, No. 2
0099-2240/07/$08.00+0     doi:10.1128/AEM.01445-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Occurrence of Virulence and Antimicrobial Resistance Genes in Escherichia coli Isolates from Different Aquatic Ecosystems within the St. Clair River and Detroit River Areas

Katia Hamelin,^1,2 Guillaume Bruant,³ Abdel El-Shaarawi,⁴ Stephen Hill,⁴ Thomas A. Edge,⁴ John Fairbrother,³ Josée Harel,³ Christine Maynard,¹ Luke Masson,¹ and Roland Brousseau^1,2*

Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Ave., Montreal, Quebec, Canada H4P 2R2,¹ Department of Microbiology and Immunology, Université de Montréal, 2900 boul. Edouard Montpetit, Montreal, Quebec, Canada H3T 1J4,² Groupe de Recherche sur les Maladies Infectieuses du Porc, Faculté de Médecine Vétérinaire, Université de Montréal, 3200 rue Sicotte, Saint-Hyacinthe, Quebec, Canada J2S 7C6,³ National Water Research Institute, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6⁴

Received 22 June 2006/ Accepted 27 October 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
Although the number of Escherichia coli bacteria in surface waters can differ greatly between locations, relatively little is known about the distribution of E. coli pathotypes in surface waters used as sources for drinking or recreation. DNA microarray technology is a suitable tool for this type of study due to its ability to detect high numbers of virulence and antimicrobial resistance genes simultaneously. Pathotype, phylogenetic group, and antimicrobial resistance gene profiles were determined for 308 E. coli isolates from surface water samples collected from diverse aquatic ecosystems at six different sites in the St. Clair River and Detroit River areas. A higher frequency (48%) of E. coli isolates possessing virulence and antimicrobial resistance genes was observed in an urban site located downstream of wastewater effluent outfalls than in the other examined sites (average of 24%). Most E. coli pathotypes were extraintestinal pathogenic E. coli (ExPEC) pathotypes and belonged to phylogenetic groups B2 and D. The ExPEC pathotypes were found to occur across all aquatic ecosystems investigated, including riverine, estuarine, and offshore lake locations. The results of this environmental study using DNA microarrays highlight the widespread distribution of E. coli pathotypes in aquatic ecosystems and the potential public health threat of E. coli pathotypes originating from municipal wastewater sources.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Escherichia coli is an important cause of disease in animals and humans worldwide. Strains of E. coli can be classified as (i) commensal, (ii) intestinal pathogenic (enteric/diarrheagenic), or (iii) extraintestinal pathogenic E. coli (ExPEC) (30). E. coli pathotypes responsible for intestinal infections include enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), enterotoxigenic E. coli (ETEC), enteroaggregative E. coli (EAEC), enteroinvasive E. coli, diffusely adherent E. coli, necrotoxic E. coli, and cell-detaching E. coli (25). Although pathogenic E. coli bacteria have been more commonly recognized as intestinal pathogens, extraintestinal E. coli infections are also a major source of morbidity, mortality, and increased health costs (29). ExPEC is the most common cause of urinary tract infections and one of the leading causes of neonatal meningitis and neonatal sepsis in humans, which can lead to serious complications and death (8). Other types of extraintestinal infections include osteomyelitis, pulmonary, intra-abdominal, soft tissue, and intravascular device-associated infections (30).

Although pathogenic E. coli bacteria are known to be associated with food-borne diseases, relatively few studies have been performed to determine their distribution in environmental surface waters (6, 12, 18, 19, 23, 27). In these studies, ExPEC bacteria have been particularly neglected, as enteric pathotypes are generally targeted. In two water-related studies where ExPEC detection was included (12, 23), almost all the pathogenic E. coli isolates were characterized as ExPEC. This unexpectedly high percentage of ExPEC isolates underscores the need for additional studies of their environmental prevalence.

ExPEC isolates have been found to share a characteristic distribution within the widely used E. coli phylogenetic classification, A, B1, B2, and D, proposed by Clermont et al. (7). Most commensal E. coli isolates derive from phylogenetic group A or B1. Obligatory pathogens responsible for acute and severe diarrhea (EHEC, ETEC, and enteroinvasive E. coli) also group within the A and B1 groups, whereas the pathotypes linked to chronic and mild diarrhea (EPEC, EAEC, and diffusely adherent E. coli) are distributed across all the four phylogenetic groups (11, 14, 15). In contrast, ExPEC isolates derive predominantly from E. coli phylogenetic group B2 and, to a lesser extent, group D (14, 15).

Microarray technology offers the most rapid and practical tool for detecting the presence or absence of a large set of virulence genes simultaneously within a given E. coli isolate. The oligonucleotide microarray used in the present study carries 348 probes, representing 189 virulence and virulence-related genes from all known E. coli pathotypes and 30 antimicrobial resistance genes (4), giving a comprehensive picture of the virulence-related gene profile (hence the pathotype) as well as the antimicrobial resistance gene profile of any given isolate. Published data demonstrate that, regardless of phylogenetic group, the virulence factor profile of an E. coli isolate predicts its in vivo pathogenicity in animal models of extraintestinal infection, providing evidence that virulence factor testing can be used to infer actual virulence potential (13). In addition, it has been suggested that the number of ExPEC virulence genes in an E. coli isolate is proportional to its pathogenic potential (28).

In the present study, E. coli bacteria were isolated from surface water samples at six locations in the St. Clair River and Detroit River areas, representing diverse fecal pollution influences ranging from an urban municipal wastewater-affected site to a relatively unpolluted site several kilometers offshore in Lake St. Clair. The four primary sampling locations also represented different aquatic ecosystems, ranging from nearshore and offshore lake environments to an urban riverine environment and a rural estuarine environment. Results showed that the distribution of pathotypes and of antimicrobial resistance genes in E. coli isolates differed significantly between sampling locations and that the E. coli pathotype most commonly encountered was ExPEC.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Study area.
Water samples were collected from four primary sampling locations, chosen to represent different aquatic ecosystems and fecal pollution influences: site 1, nearshore Lake Huron water at the mouth of the St. Clair River, collected several hundred meters from the shoreline; site 2, offshore Lake St. Clair water collected several kilometers from the shoreline; site 3, estuarine water from the mouth of the Thames River, collected downstream from a rural agricultural landscape; and site 4, riverine water from the Detroit River, just downstream from the City of Detroit and the mouth of the Rouge River. Sites 1 and 2 were selected to represent relatively unpolluted sites without significant fecal pollution sources known to be nearby. Site 3 was anticipated to be influenced by diverse upstream sources of fecal pollution from agricultural activities, small towns, and wildlife, although there were marshlands in the immediate area near the sampling location. Site 4 was selected to represent a location likely affected by urban municipal wastewater sources from combined sewer and sanitary sewer outfalls and the discharge of a sewage treatment plant. The two secondary sampling locations were site 5, the middle of the Detroit River, offshore from downtown Detroit, and site 6, the Canadian side of the Detroit River, just downstream of Fighting Island, where a large colony of sea gulls resides. The secondary sampling locations were selected to explore the possibility of unique virulence and antimicrobial resistance patterns for sites representing surface water runoff from a densely populated urban environment and bird-affected waters, respectively. Characteristics of the different sampling locations are shown in Table 1 and their geographical positions shown in Fig. 1.

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TABLE 1. Description of the sampling locations in different aquatic ecosystems within the St. Clair River and Detroit River areas

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FIG. 1. Map of St. Clair River and Detroit River areas showing the locations of the sampling sites. Sites are labeled as described for Table 1.

Water sampling and E. coli isolation.
Water samples were collected in sterile 250-ml or 1-liter polypropylene bottles on 26 and 27 July 2005 onboard the research vessel Limnos. The samples were collected using an ISOMET sampler (21) that permitted the opening of water bottles away from the hull, at a depth of about 0.5 m below the surface of the water. Other water samples, including those from the Lake St. Clair offshore site, were collected by a smaller boat launched from the Limnos. Ten to 12 water samples were collected at each site and processed immediately onboard the vessel. Volumes of between 100 and 300 ml of water were filtered through 0.45-µm membrane filters (up to 12 liters were processed for some sites). The filters were incubated overnight at 44.5°C on differential coliform agar supplemented with lactose, the chromogenic substrate 5-bromo-6-chloro-3-indolyl glucuronide (BCIG), and cefsulodin (Oxoid Inc.). Up to 10 positive colonies from each water sample were randomly picked with a sterile toothpick and streaked onto MacConkey agar (Difco Inc.) for overnight growth at 37°C. Putative E. coli isolates were tested for glucuronidase activity by growth and fluorescence in E. Coli broth with 4-methylumbelliferyl-ß-glucuronide (Difco Inc.) and for indole production by growth in 1% (wt/vol) tryptone (Difco Inc.) and reaction with Kovac's reagent (Oxoid Inc.). Isolates positive for both tests were stored at –80°C in tryptic soy broth and 15% (vol/vol) glycerol. E. coli ATCC 29194 and Klebsiella pneumoniae ATCC 33495 were used as positive and negative controls, respectively, during confirmatory tests.

DNA extraction.
E. coli isolates were grown overnight in 5 ml of tryptic soy broth at 37°C. One milliliter of cell suspension for each isolate was transferred to 1.5-ml centrifuge tubes and spun at 15,500 x g for 2 min. The supernatants were removed and the cell pellets resuspended in 200 µl of sterile water by vortexing. The suspensions were boiled for 10 min and centrifuged as before, and 150 µl of each supernatant containing DNA was removed for testing.

E. coli DNA labeling.
Bacterial DNA was labeled using the Bioprime DNA labeling system (Invitrogen Life Technologies, Burlington, Ontario, Canada). In order to remove any contaminating RNA, 2 µl of RNase A (10 mg/ml) (USB, Cleveland, OH) was added to the genomic DNA sample for 2 min at 24°C, followed by centrifugation (15,500 x g, 2 min, 24°C). Four microliters of the supernatant was added to a final volume of 50 µl containing 20 µl of random-primer solution, 1 µl of high-concentration DNA polymerase (Klenow fragment, 40 U/µl), 5 µl of a deoxynucleoside triphosphate mix (1.2 mM dATP, 1.2 mM dGTP, 1.2 mM dTTP, and 0.6 mM dCTP in 10 mM Tris [pH 8.0] and 1 mM EDTA), and 2 µl of 1 mM Cy5-dCTP. Labeling reactions were performed in the dark at 37°C for 3.5 h and stopped by addition of 5 µl of 0.5 M Na₂-EDTA (pH 8.0). After addition of 2.5 µl of 3 M sodium acetate (pH 5.2), the labeled samples were purified with a PureLink PCR purification kit (Invitrogen Life Technologies) according to the manufacturer's protocol. The amount of incorporated fluorescent Cy5 dye was then quantified by scanning the DNA sample from 200 to 700 nm and subsequently inputting the data into the Internet-based percent incorporation calculator found at http://www.pangloss.com/seidel/Protocols/percent_inc.html.

DNA microarray.
The microarray used in this study is based on earlier published work (2), with the addition of recently identified virulence-related genes in E. coli and the antimicrobial resistance genes most commonly encountered in gram-negative bacteria (20). The current version carries 348 oligonucleotides (70 mers) targeting 189 virulence or virulence-related genes representing all known E. coli pathotypes and 30 antimicrobial resistance genes (4). The microarray also carries four positive-control DNAs for E. coli derived from the tryptophanase (tnaA), beta-glucuronidase (uidA), lactose permease (lacY), and beta-galactosidase (lacZ) gene sequences. Negative controls consist of oligonucleotides derived from the green fluorescent protein gene sequence of Aequoria victoria and the chlorophyll synthase gene from Arabidopsis thaliana. Microarray construction, validation, and oligonucleotide probe sequence information has been published elsewhere (4).

Hybridization of labeled DNA.
Hybridization was performed as described in an earlier publication (12). In summary, microarrays were prehybridized at 50°C for 1 h, followed by hybridization overnight at 50°C, in DIG Easy Hyb buffer (Roche Diagnostics, Laval, Quebec, Canada) containing 5% (vol/vol) bovine serum albumin. After hybridization, the microarrays were processed through four stringency washes (three in 0.1x SSC [1x SSC is 0.15 M NaCl plus 0.015 M sodium citrate], 0.1% [wt/vol] sodium dodecyl sulfate and one in 0.1x SSC) at 37°C for 5 min under agitation. The slide was then air dried and scanned at a resolution of 10 µm at 85% laser power with a ScanArray Lite fluorescent microarray analysis system (Perkin-Elmer, Missasauga, Ontario, Canada). Acquisition of fluorescent spots and quantification of fluorescent spot intensities were performed using ScanArray Express software version 2.1 (Perkin-Elmer, Foster City, CA). All data were normalized by subtracting local background values from the recorded spot intensities for one subarray. For each subarray, the median value for each set of triplicate-spotted oligonucleotides was compared to the median value for the subarray from which the triplicate set of spots was derived. Oligonucleotides with a signal-to-noise fluorescence ratio greater than 2.0 were considered positive.

Pathotype and phylogenetic group classification.
Each E. coli isolate was assigned to a particular pathotype (17, 24, 25) according to its set of virulence genes or markers: for ETEC, LT and/or STa and/or STb; for STEC subgroup EHEC, stx₁ and/or stx₂, espA, espB, tir, and eae; for STEC-potential EHEC, stx₁ and/or stx₂; for EAEC, capU, shf, and virK, aggregative adherence fimbria-encoding genes; for atypical EPEC, espA, espB, tir, eae, and its variants and the absence of bfpA; for ExPEC subgroup uropathogenic E. coli (UPEC), P pilus-encoding gene, hlyA, and S fimbria-encoding genes, chuA, fepC, cnf1, irp1, irp2, fyuA, iroN, and usp; for ExPEC subgroup septicemia-associated E. coli (SEPEC), cdtB-3, cdtB-2, cdtB-1, cdtB-4, cnf2, F17A, f165(1)A, iucD, and gafD; for ExPEC subgroup meningitis-associated E. coli (MNEC), ibeA, neuA, and neuC; and for "other ExPEC," kpsM, iutA, iucD, traT, malX, irp1, irp2, fyuA, chuA, fepC, iss, and kfiB (25). Isolates lacking one or more of the genes defining a given set were considered nonpathogenic. E. coli isolates were assigned to a phylogenetic group based on the presence of the chuA, TspE4.C2, and yjaA genes as described by Clermont et al. (7).

Statistical analysis.
The likelihood ratio test (16) was used to assess the significance of comparative results under the binomial and multinomial models.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Virulence genes associated with ExPEC are not as defined as those for enteric pathotypes, and classification can be somewhat arbitrary. For the purposes of this study, we have defined "other ExPEC" as E. coli isolates carrying ExPEC-related genes without being assignable to one of the three defined ExPEC classes, UPEC, MNEC, or SEPEC (12). This category includes commensal E. coli bacteria possessing virulence genes which may have been acquired through genetic exchanges (15) and UPEC bacteria that may have lost genes located on pathogenicity islands, as some pathogenicity islands become unstable at temperatures lower than 21°C (22).

Site-specific distribution of 308 E. coli isolates according to pathotype.
The percentages of E. coli isolates having defined pathotypes for the six sampling locations are shown in Table 2. The percentages of isolates with defined pathotypes were significantly higher for sites 4 and 5 (near Detroit), which were anticipated to be more influenced by urban municipal wastewater than other locations. For the four primary locations, there was a greater percentage of E. coli isolates with defined pathotypes for site 4 (48%) than for sites 1 (27%), 2 (20%), and 3 (27%). These differences are highly significant (P < 0.003). The two secondary sites were located close to site 4 (downstream from the city of Detroit), so results similar to those obtained for site 4 might have been expected. However, whereas site 5 had almost the same proportion of isolates with defined E. coli pathotypes as site 4 (50%), only 18% of the isolates from site 6 (near Fighting Island) had pathogenic E. coli pathotypes.

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TABLE 2. Percentages of E. coli pathotypes among isolates from different aquatic ecosystems within the St. Clair River and Detroit River areas

Most pathotype isolates (85 out of 117) were classified as ExPEC, which was found to occur across all aquatic ecosystems investigated, including riverine, estuarine, and offshore lake locations. A relatively low percentage of enteric pathotypes was found, except at site 2 (in the middle of Lake St. Clair), where enteric pathotypes were found more frequently than ExPEC (Table 2). These results are consistent with two other studies that demonstrated relatively high percentages of ExPEC in surface waters (12, 23). No EHEC bacteria were detected in water samples, atypical EPEC and EAEC being the only intestinal pathotypes found in our study. Site 4 (urban) demonstrated a large diversity of ExPEC pathotypes, including UPEC and SEPEC, and was the only one where an MNEC pattern was observed. In comparison, site 1 (in Lake Huron) possessed the least diversity of pathotypes. Although site 5 demonstrated the same overall percentage of E. coli pathotypes as site 4, only the pathotype "other ExPEC" was found in the latter.

It is noteworthy that 25% of all of the E. coli isolates carried unusual virulence gene combinations (that do not correspond to a specific pathotype) and/or at least three antimicrobial resistance genes, yet these were not classified as pathotypes by our criteria. The frequency of E. coli pathotypes reported in this study can therefore be considered a conservative estimate.

Phylogenetic group distribution.
The percentages of E. coli isolates in each phylogenetic group differed according to the location (Table 3). Sites 1 and 2 showed similar distributions among the different phylogenetic groups, with most E. coli isolates belonging to groups A and B1 and only a few belonging to groups D and B2. At site 3, only 2% of the isolates were classified in group A, whereas 65% belonged to group B1. The isolates were almost equally distributed among the four phylogenetic groups at site 4, which was the only site where a substantial percentage of isolates belonged to group B2. On the other hand, the secondary location site 5 showed a higher percentage of E. coli isolates in group D (80%), whereas site 6 showed a majority of isolates belonging to group A (64%), although the sample sizes were small.

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TABLE 3. Frequencies of phylogenetic groups among E. coli isolates from different aquatic ecosystems within the St. Clair River and Detroit River areas

The distribution of the pathotypes among the phylogenetic groups (Table 4) was consistent with other published data (14). Indeed, E. coli isolates that were not pathotypes primarily belonged to groups A and B1, whereas ExPEC isolates are found in group B2 and, to a lesser extent, group D. On the other hand, atypical EPEC (24) and EAEC isolates were distributed among the various groups (11, 14, 15). It is noteworthy that only one E. coli isolate belonging to group B2 was not classified as a pathotype.

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TABLE 4. Frequencies of pathotypes and antimicrobial resistance genes among E. coli isolates from different aquatic ecosystems within the St. Clair River and Detroit River areas

Site-specific distribution of E. coli isolates possessing antimicrobial resistance genes.
Based on the frequency of antimicrobial resistance genes, the six sites were divided into three sets (P < 0.001). The highest frequency of antimicrobial resistance genes occurred at site 4 and the second highest at site 1, whereas the other sites formed a statistically homogeneous set (Table 5). Downstream from the city of Detroit (site 4), 27% of E. coli isolates were found to carry at least one antimicrobial resistance gene, compared to 18% for site 1. Null or very low percentages of E. coli isolates carrying antimicrobial resistance genes were found at sites 2 (0%) and 3 (3%). The two secondary locations showed 0% (site 5) and 9% (site 6) of E. coli isolates carrying antimicrobial resistance genes.

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TABLE 5. Frequencies of antimicrobial resistance gene-positive E. coli isolates belonging to different pathotypes in different aquatic ecosystems within the St. Clair River and Detroit River areas

The antimicrobial resistance genes most frequently detected were tet(A) and tet(B) (44% and 28%, respectively, among E. coli isolates carrying resistance genes), bla_TEM (53%), and sulII (39%), which code for resistance to tetracycline, ampicillin, and sulfonamide, respectively (Table 6).

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TABLE 6. Frequencies of antimicrobial resistance genes among 308 E. coli isolates from different aquatic ecosystems within the St. Clair River and Detroit River areas

Site-specific distribution of E. coli isolates possessing a tsh gene.
The tsh gene was tested across sampling locations as a possible marker for E. coli originating from an avian source (Table 7) (9, 33). The greatest frequency of E. coli isolates carrying the tsh gene was found at site 3 (in the estuary at the mouth of the Thames River) (17%), followed by site 1 (in nearshore Lake Huron water) (11%) and site 2 (in offshore Lake St. Clair water) (9%). No E. coli bacteria carrying the tsh gene were found at urban site 4 or at site 6. Urban site 5 showed approximately 30% tsh-positive isolates, although the sample size at this site was small. Except for those from site 2, most E. coli isolates carrying the tsh gene did not have defined pathotypes.

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TABLE 7. Frequencies of tsh gene-positive E. coli isolates belonging to different pathotypic groups in various aquatic ecosystems within the St. Clair River and Detroit River areas

Top Abstract Introduction Materials and Methods Results Discussion References

 
This study investigated the distribution of defined pathotypes and antimicrobial-resistant E. coli bacteria in water samples from the St. Clair River and Detroit River areas. The transboundary waters in this area are an important shared resource for millions of people in Canada and the United States. Water quality is a significant concern for many communities that rely on these source waters for drinking and recreation. A number of binational Areas of Concern currently identify beach closures as important water use impairments, and concern has been raised about pathogen pollution and the spread of antibiotic-resistant bacteria in the Great Lakes basin ecosystem (1).

The results show that ExPEC bacteria were found to occur at every aquatic ecosystem location that we sampled. Overall, the site most influenced by urban municipal wastewater (site 4) demonstrated the highest percentage of defined E. coli pathotypes as well as E. coli bacteria carrying antimicrobial resistance genes. At this site, ExPEC bacteria classified as MNEC, UPEC, and SEPEC (associated with meningitis, urinary tract infection, and septicemia, respectively) were found. Moreover, intestinal E. coli pathotypes were also observed. The ExPEC isolates belonged to both the D and the B2 phylogenetic groups, the latter being the predominant group from which ExPEC bacteria are derived (14). While this study did not determine the specific source of E. coli isolates, it is likely that the isolates from site 4 were from nearby combined sewer overflows or sewage treatment plant outfalls, implicating municipal wastewater as a potential source for E. coli pathotypes. Another interesting finding was the higher percentage of E. coli pathotypes carrying antimicrobial resistance genes at site 4. This may reflect the influence of human antibiotic use, again suggesting the importance of municipal wastewater as a potential source of antimicrobial-resistant E. coli pathotypes.

Fairly high percentages of E. coli isolates possessing defined pathotypes (27%) and of E. coli isolates carrying resistance genes (18%) were found at site 1, which was anticipated to be relatively unpolluted. Site 1 was several hundred meters out from the shoreline and away from obvious nearby fecal pollution point sources. While the site had low numbers of E. coli bacteria, the relatively high frequency of antimicrobial resistance was unexpected. It is possible that these E. coli bacteria in the nearshore waters were able to persist and be transported by alongshore currents from unknown human or agricultural sources.

While the lack of antimicrobial resistance genes was anticipated for site 2 (several kilometers offshore in Lake St. Clair), the relatively high frequency of intestinal E. coli pathotypes was unexpected. About 97% of the water entering Lake St. Clair comes from the St. Clair River, with the rest provided by the two watersheds on each side of the lake (32). The presence of the deep shipping channel that divides the lake encourages water to flow right through from the St. Clair River to the Detroit River without being retained. As a result, the lake's water is completely exchanged every 5 to 7 days (32). It is possible that the E. coli bacteria collected at site 2 were recently deposited from boat or bird sources (though the low occurrence of the tsh avian marker argues against the latter) or that they were able to persist and be transported relatively quickly offshore from fecal pollution sources, such as municipal wastewater outfalls in the St. Clair River.

The Thames River drains extensive agricultural lands in southern Ontario, and it was originally anticipated that site 3 might reflect a more agriculture-based fecal pollution influence. However, at the time of sample collection, water flow was very slow near the river mouth, and the environment immediately around the sampling location consisted more of marshes along the river banks than agricultural land runoff. It is possible that fecal pollution at the site might have originated from more local sources, such as birds and other wildlife species along the river banks, which would be consistent with the relatively high frequency of the tsh gene and the relatively low frequency of antimicrobial resistance genes found in E. coli isolates at this site. The tsh gene has been suggested as a possible marker for indicating the presence of fecal pollution from an avian source, like waterfowl (12, 26, 33). A number of studies have found E. coli bacteria from wildlife exhibiting relatively low prevalences of antimicrobial resistance compared to those from human or agricultural sources (10, 31).

Results showing that tet(A), tet(B), bla_TEM, and sulII are the most abundant antimicrobial resistance genes (Table 6) are consistent with those for two other studies demonstrating similar antimicrobial gene occurrences in the environment (12, 31). In our study, 28% of the E. coli isolates that carried resistance genes showed the presence of a class 1 integron and also carried two or more antimicrobial resistance genes. Approximately similar numbers of E. coli isolates carrying resistance genes were found in all phylogenetic groups except for group D, which included only one isolate carrying a resistance gene (Table 4).

The phylogenetic classification of the E. coli isolates for our study showed a good agreement with those for other studies (14, 15). Table 4 clearly shows that there is a link between phylogenetic group and occurrence of virulence genes. Nonpathotype isolates were mostly in groups A and B1, whereas extraintestinal E. coli isolates were predominantly in phylogenetic group B2 and, to a lesser extent, group D. The difference between groups D and B2 was clearly shown, as group B2 contained only E. coli pathotypes (except for one isolate) and group D included both pathotype and nonpathotype E. coli. Furthermore, the results were consistent with our use of the classification "other ExPEC" pathotype, as these isolates mostly belonged to groups D and B2. Our results also confirmed certain findings reported by others, such as the presence of the sfa-foc operon only in the phylogenetic group B2 (3; data not shown). The high percentage of ExPEC belonging to the B2 phylogenetic group has also been observed in previous studies where these isolates were associated with both human and nonhuman mammalian extraintestinal infections (5).

The occurrence of isolates not classified as pathotypes yet possessing a subset of virulence genes could be explained by the high genome plasticity of E. coli. This plasticity exists due to the dynamic genetic exchange of virulence genes through plasmids, pathogenicity islands, and other mobile genetic elements. These genetic factors contribute to the rapid evolution of E. coli strains and to the formation of unusual virulence gene combinations that could potentially lead to the evolution of new pathotypes. Further studies are needed to verify whether some of these unusual patterns demonstrate pathogenicity in tissue culture or animal models.

In conclusion, we found ExPEC in diverse aquatic ecosystems and at all six locations that were sampled in the St. Clair River and Detroit River areas, using our microarray. There was considerable variation in the frequencies of virulence and antimicrobial resistance genes in E. coli isolates from the different sites. However, the occurrence of the highest frequency of E. coli bacteria with defined pathotypes in surface waters most likely affected by municipal wastewater sources underscores the importance of managing combined sewer and sanitary sewer overflows and sewage treatment plant effluents. Although ExPEC bacteria are commonly found as commensals in healthy humans (29), our finding that the majority of the E. coli pathotypes in aquatic ecosystems were ExPEC remains a potential public health concern, as E. coli bacteria of this pathotype are responsible for an estimated 40,000 deaths and expenditure of at least $2.6 billion annually in the United States alone (29). Therefore, our data clearly indicate a need to further investigate the occurrence of pathogenic E. coli, especially ExPEC, in source waters used for drinking, recreation, and irrigation in order to better understand the implications for public health.

 
We thank Charlotte Curtis, Bailey Davis, Alyssa Loughborough, Kinga Smolen, Flora Suen, and the crew of the Limnos for assistance in sample collection. We give particular thanks to Bailey Davis for also assisting with DNA extractions.

Funding was provided by the Great Lakes 2020 program and the Canadian Water Network.

 
* Corresponding author. Mailing address: Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Ave., Montreal, Quebec, Canada H4P 2R2. Phone: (514) 496-6152. Fax: (514) 496-6213. E-mail: Roland.Brousseau{at}cnrc-nrc.gc.ca.

Published ahead of print on 3 November 2006.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Applied and Environmental Microbiology, January 2007, p. 477-484, Vol. 73, No. 2
0099-2240/07/$08.00+0     doi:10.1128/AEM.01445-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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