Impact of an urban effluent on antibiotic resistance of riverine Enterobacteriaceae and Aeromonas spp

In order to evaluate the impact of an urban effluent on antibiotic resistance of freshwater bacterial populations, water samples were collected from the Arga river (Spain), upstream and downstream from the wastewater discharge of the city of Pamplona. Strains of Enterobacteriaceae (representative of the human and animal commensal flora) (110 isolates) and Aeromonas (typically waterborne bacteria) (118 isolates) were selected for antibiotic susceptibility testing. Most of the Aeromonas strains (72%) and many of the Enterobacteriaceae (20%) were resistant to nalidixic acid. Singly nalidixic acid-resistant strains were frequent regardless of the sampling site for Aeromonas, whereas they were more common upstream from the discharge for enterobacteria. The most common resistances to antibiotics other than quinolones were to tetracycline (24.3%) and beta-lactams (20.5%) for Enterobacteriaceae and to tetracycline (27.5%) and co-trimoxazole (26.6%) for Aeromonas. The rates of these antibiotic resistances increased downstream from the discharge at similar degrees for the two bacterial groups; it remained at high levels for enterobacteria but decreased along the 30-km study zone for Aeromonas. Genetic analysis of representative strains demonstrated that these resistances were mostly (enterobacteria) or exclusively (Aeromonas) chromosomally mediated. Moreover, a reference strain of Aeromonas caviae (CIP 7616) could not be transformed with conjugative R plasmids of enterobacteria. Thus, the urban effluent resulted in an increase of the rates of resistance to antibiotics other than quinolones in the riverine bacterial populations, despite limited genetic exchanges between enterobacteria and Aeromonas. Quinolone resistance probably was selected by heavy antibiotic discharges of unknown origin upstream from the urban effluent.     

Chromate-resistance genes in plasmids from antibiotic-resistant nosocomial enterobacterial isolates

The presence of chromate-resistance genes in enterobacteria was evaluated in a collection of 109 antibiotic-resistant nosocomial isolates from nine major cities in México. Results were compared with the presence of mercury-resistance genes. Susceptibility tests showed that 21% of the isolates were resistant to chromate (Cr(R)), whereas 36% were resistant to mercury (Hg(R)). Cr(R) levels were high in Klebsiella pneumoniae (61%), low in Enterobacter cloacae (12%) and Escherichia coli (4%), and null in Salmonella sp. isolates. Colony hybridization demonstrated that the majority of metal-resistant isolates hybridized with chrA gene (87% of Cr(R) isolates), encoding a CHR transporter homologue, and merA gene (74% of Hg(R) isolates), encoding MerA mercuric reductase, suggesting that most isolates expressed these widespread metal-resistance systems. Southern blot hybridization of Cr(R) isolates showed that plasmids of 80, 85, and 95 kb from K. pneumoniae isolates, and of 100 kb from an E. cloacae isolate, contained chrA-related sequences. These plasmids belonged to IncN or IncP incompatibility groups, and conferred Cr(R), as well as multiple antibiotic resistance, when transferred by conjugation to an E. coli standard strain. These data indicated that Cr(R) genes may be distributed among clinical enterobacteria via conjugative plasmids, probably by coselection with antibiotic-resistant genes.     

Taxonomic spectrum and antibiotic resistance of Enterobacteriaceae isolated from bile of patients with cholangitis]

The data on the taxonomic structure and antibiotic resistance of Enterobacteriaceae isolated from bile of patients with various clinical variants of cholangitis are presented. It was shown that bacteriocholia was registered in 53.3-90.9% of the patients during operations and in 88.9-100% of the patients during the postoperative period. Among bilicultures enterobacteria dominated (93.4% of the cases) with predominance of E. coli and Klebsiella spp. (the total about 70%). The enterobacteria isolates were frequently resistant to ampicillin and amoxycillin/clavulanate (92.6 and 70.4% of the strains respectively), gentamicin and amikacin (74.1 and 22.2%), cefazolin and cefotaxime (88.9 and 37.0%). The resistance to imipenem and ciprofloxacin (7.4%) was rare, that should be considered while prescribing drugs for empirical antimicrobial chemotherapy.     

Occurrence and antimicrobial resistance of Gram-negative bacteria isolated in haemodialysis water and dialysate of renal units: results of a Greek multicentre study

AIMS: To evaluate the occurrence, identity and antimicrobial resistance of Gram-negative bacteria isolated from municipal water supplies, treated water, and dialysate of all 85 Greek haemodialysis centres. METHODS AND RESULTS: A total of 141 Gram-negative bacterial isolates (98 non-fermentative and 43 enterobacteria) were recovered from 255 water samples. Twenty-four of them were isolated from tap water, 31 from treated water, and 86 from dialysate samples. The mean concentrations (CFU per 100 ml +/- s.d.) of the positive Gram-negative bacteria samples were 69.2 +/- 43.9, 31.2 +/- 28.7 and 3552.3 +/- 4485.0, respectively. The most common isolates, in order of frequency were Pseudomonas aeruginosa (22.7%), Chryseobacterium meningosepticum (14.9%), Stenotrophomonas maltophilia (13.5%), Escherichia coli (12.8%) and Enterobacter cloacae (7.8%), representing 71.6% of all isolates. Ps. aeruginosa was the most prevalent isolate in all types of water sample followed by C. meningosepticum in tap and treated water and by E. coli in dialysate. Nineteen per cent of the enterobacteria and 35% of the non-fermenters were resistant against three or more of the nine antibiotics tested. CONCLUSIONS: These data suggest that dialysate and treated water could be a source of infection for several non-fermentative and enterobacterial species. IMPACT OF THE STUDY: Microbiological monitoring of such samples is needed in order to know the identity and antibiotic resistance profiles of their potentially pathogenic bacterial population.     

Diverse geno- and phenotypes of persistent Listeria monocytogenes isolates from fermented meat sausage production facilities in Portugal

The persistence of Listeria monocytogenes in food-associated environments represents a key factor in transmission of this pathogen. To identify persistent and transient strains associated with production of fermented meat sausages in northern Portugal, 1,723 L. monocytogenes isolates from raw material and finished products from 11 processors were initially characterized by random amplification of polymorphic DNA (RAPD), PCR-based molecular serotyping, and epidemic clone characterization, as well as cadmium, arsenic, and tetracycline resistance typing. Pulsed-field gel electrophoresis (PFGE) typing of 240 representative isolates provided evidence for persistence of L. monocytogenes for periods of time ranging from 10 to 32 months for all seven processors for which isolates from different production dates were available. Among 50 L. monocytogenes isolates that included one representative for each PFGE pattern obtained from a given sample, 12 isolates showed reduced invasion efficiency in Caco-2 cells, including 8 isolates with premature stop codons in inlA. Among 41 isolates representing sporadic and persistent PFGE types, 22 isolates represented lysogens. Neither strains with reduced invasion nor lysogens were overrepresented among persistent isolates. While the susceptibility of isolates to lysogenic phages also did not correlate with persistence, it appeared to be associated with molecular serotype. Our data show the following. (i) RAPD may not be suitable for analysis of large sets of L. monocytogenes isolates. (ii) While a large diversity of L. monocytogenes subtypes is found in Portuguese fermented meat sausages, persistence of L. monocytogenes in this food chain is common. (iii) Persistent L. monocytogenes strains are diverse and do not appear to be characterized by unique genetic or phenotypic characteristics.     

Biological properties of some nitrogen-fixing associative enterobacteria

Enterobacteria isolates from potato tubers were able to fix nitrogen, to protect plants against phytopathogens and to produce phytohormones thus increasing the plant yield. These isolates were previously phenotypically identified as Erwinia carotovora; however, they differed from typical E. carotovora in a number of biological characteristics and were found to be nonphytopathogenic (avirulent) due to the lack of pectate lyase activity. A data matrix, containing 31 strains and 105 biological characteristics was used for computer cluster analysis. The avirulent strains formed a separate cluster more closely related to Klebsiella spp. strains (with a 0.67 level of similarity) than to typical phytopathogenic bacteria of the E. carotovora group (with a 0.48 level of similarity). A phylogenetic analysis based on restriction polymorphisms of an amplified ribosomal DNA spacer region revealed that the avirulent strains studied here were different from all Erwinia, Klebsiella and other enterobacteria species strains. The AP PCR/hybridization technique showed cross homology of amplified DNA of these avirulent strains and a lack of such homology with the DNA from strains of other species. Numerical taxonomy data, rDNA analysis and AP PCR/hybridization assays confirmed that these avirulent bacteria may be regarded as an independent group of enterobacteria.     

Characteristics of the community-genotype sequence type 72 methicillin-resistant <Emphasis Type="Italic">Staphylococcus aureus</Emphasis> isolates that underlie their persistence in hospitals

Panton-Valentine leukocidin-negative methicillin-resistant Staphylococcus aureus (MRSA) clone ST72, known as a major community-associated MRSA in Korea, has emerged as an important pathogen in hospitals. To understand bacterial properties that underlie transformation of this clone into a nosocomial pathogen, we compared characteristics of the community-genotype ST72 MRSA isolates with those of ST5 and ST239 MRSA, which have been predominant nosocomial MRSA clones in Korea. Several genes associated with adhesion and virulence were absent or rarely found in ST72 isolates. Many ST72 isolates (70.1%) belonged to agr group I, but the agr group of other ST72 isolates could not be determined. As indicated by d-hemolysin production, ST72 isolates expressed fully functional agr, whereas agr dysfunction was observed in ST5 and ST239 isolates. In the biofilm formation assay, no upregulation of biofilm-forming activity of ST72 MRSA was detected. However, ST72 isolates demonstrated persistence under hypotonic and desiccating conditions (survival rates 72.3% and 33.9%, respectively), which was similar to characteristics of ST5 or ST239 isolates. ST72- MRSA isolates showed low virulence, but properties of their functional agr system could facilitate their spread in hospitals. In conclusion, tolerance to stressful environments, e.g., hypotonic and dry conditions, may also contribute to survival of the community-associated MRSA clones in healthcare facilities.     

PCR genotyping of Acinetobacter hospital isolates

A total of 13 Acinetobacter baumannii-13TU isolates obtained from patients of the Perm regional clinical hospital during the period of 1 year, were genotyped in the polymerase chain reaction (PCR) with universal primers. Acinetobacter cultures could not be distinguished by phenotypic tests and antibiogram and were resistant to B-lactams and gentamicin. According to the results of amplification the obtained isolates could be subdivided into two groups (with 6 and 7 respectively). The detection of "epidemic" strains, including those capable of prolonged (for more than 5 months) persistence in hospital environment, may be indicative of the growing role of Acinetobacter as the causative agent in nosocomial infection.     

Extrapulmonary Tuberculosis: Mycobacterium tuberculosis Strains and Host Risk Factors in a Large Urban Setting in Brazil

Background

Factors related to the development of extrapulmonary forms of tuberculosis (EPTB) are still poorly understood, particularly in high-endemic countries like Brazil. The objective of the paper is to determine host and Mycobacterium tuberculosis (MTB) strain-related factors associated with the development of EPTB in Espírito Santo state, Brazil.

Methods and Findings

We conducted a retrospective laboratory-based surveillance study of new tuberculosis (TB) cases diagnosed in Espírito Santo state, Brazil between 1998 and 2007. We genotyped 612 isolates of MTB from 606 TB patients using spoligotyping and IS6110-restriction fragment length polymorphism (RFLP) typing and compared sociodemographic and clinical characteristics of patients with pulmonary TB (PTB) and EPTB. Among 606 patients, 464 (77%) had PTB, 79 (13%) had EPTB, 51 (8%) had both, and 12 (2%) had miliary TB. The IS6110 RFLP analysis demonstrated that 250 (41%) isolates belonged to clustered RFLP patterns, 27 (11%) of which were from EPTB. We identified 73 clusters including 35 (48%) composed of 2 isolates each. By spoligotyping, 506 (83%) MTB isolates fell into known patterns and 106 (17%) fell into patterns with no family assignment; 297 (48%) isolates belonged to the Latin-American Mediterranean family. Higher school level (4-7 years OR: 0.16 95% CI 0.34-0.73 and > 8 years of education, OR 0.06 95% CI 0.009-0.50) white ethnicity (OR: 2.54 95% CI 1.03-6.25) and HIV infection (OR: 16.83 95% CI 5.23-54.18) were associated with EPTB. No specific strain lineage or percentage of clustering was associated with EPTB.

Conclusions

These results demonstrate that risk factors for EPTB are related more to host than to MTB strain lineage characteristics.     

Dissemination of resistance plasmids among gentamicin-resistant enterobacteria from hospital patients.

Out of 184 patients who were infected or colonised with gentamicin-resistant enterobacteria, 17 (9%) harboured more than one strain. Single antibiotic-resistance plasmids were common to more than one of the different organisms isolated from nine patients, strongly suggestive of in-vivo conjugation. An "epidemic" plasmid with a molecular weight of approximately 110 megadaltons was found in 11 distinct strains isolated from four patients. Seven of the organisms harbouring this plasmid were Klebsiella aerogenes. Spread of multiple-resistance plasmids among endemic gentamicin-resistant enterobacteria is not uncommon, and these organisms provide a reservoir of plasmids that may ultimately spread to more pathogenic genera.     

Drug resistance of opportunistic enterobacteria isolated from various sources]

Resistance of 159 strains of opportunistic enterobacteria to 9 antibacterial drugs was studied. The strains were isolated from man and cattle. It was shown that the overwhelming majority of the isolates (93 per cent) were polyresistant irrespective of the genus. There was a high frequency of the strains resistant to the widely used antibiotics such as chloramphenicol (73 per cent), ampicillin (73.6 per cent) and rifampicin (95.6 per cent) and sulfanilamides (99.3 per cent). Gentamicin and nalidixic acid proved to be the most active against the cultures: 11.9 and 10 per cent of the resistant strains, respectively. The strains of enterobacteria isolated from different sources had a sensitivity to the antibiotics. Multiple antibiotic resistance to at least 5 drugs, variability of the spectra and high resistance were more characteristic of the isolates from the animals. The necessity of a rational use of antibacterial drugs in veterinary is indicated.     

Prevalence of plasmid-mediated quinolone resistance determinants among oxyiminocephalosporin-resistant Enterobacteriaceae in Argentina

High quinolone resistance rates were observed among oxyiminocephalosporin-resistant enterobacteria. In the present study, we searched for the prevalence of plasmid-mediated quinolone resistance (PMQR) genes within the 55 oxyiminocephalosporin-resistant enterobacteria collected in a previous survey. The main PMQR determinants were aac(6'')-Ib-cr and qnrB, which had prevalence rates of 42.4% and 33.3%, respectively. The aac(6'')-Ib-cr gene was more frequently found in CTX-M-15-producing isolates, while qnrB was homogeneously distributed among all CTX-M producers.     

Human-associated extended-spectrum β-lactamase in the Antarctic

Escherichia coli bacteria with extended-spectrum β-lactamase (ESBL) type CTX-M resistance were isolated from water samples collected close to research stations in Antarctica. The isolates had bla(CTX-M-1) and bla(CTX-M-15) genotypes and sequence types (ST) indicative of a human-associated origin. This is the first record of ESBL-producing enterobacteria from Antarctica.     

Enterobacteria in feedlot waste and runoff

Samples of beef cattle feedlot waste (FLW), runoff from the pens, and water from a large drainage ditch at the feedlot were examined for Enterobacteriaceae. The drainage ditch receives the runoff but contains primarily subsurface drainage from fields on which FLW is spread for disposal. Planting and enrichment techniques with seven different media were used to isolate 553 cultures of enterobacteria. FLW contains about 50 million enterobacteria/g dry weight. More than 90% of these were Escherichia coli, none of which were enteropathogenic types as determined with multivalent sera. Citrobacter and Enterobacter cloacae were other organisms present in moderate numbers. Application of enrichment techniques broadened the spectrum of enterobacteria isolates to include the four Proteus spp., both Providencia spp., Klebsiella, Enterobacter aerogenes, Arizona, and a single isolate of Salmonella (serological group C₂). Shigella was not isolated. The wide spectrum of enterobacteria in FLW may be a hazard if unsterilized waste is refed. Fewer enterobacteria occurred in the runoff and in the drainage ditch; the most numerous species in FLW also were most numerous at these sites. However, neither Salmonella nor Arizona was isolated from runoff or drainage-ditch waters.     

Methicillin-Resistant Staphylococcus aureus in Commercial Swine Herds Is Associated with Disinfectant and Zinc Usage

Methicillin-resistant Staphylococcus aureus (MRSA) originating from swine is concerning for public health, but an understanding of the emergence and persistence of MRSA in nursery herds is lacking. The aim of this study was to determine whether MRSA in nursery pigs is associated with particular herd-level parameters, including the use of antimicrobials, disinfectants, and heavy metals, which may be driving the selection and persistence of antimicrobial resistance. Nasal cultures for MRSA were completed for 390 pigs from 26 farms at the end of the suckling phase and again at 3 weeks postweaning. Herd-level information was collected, and a random subset of MRSA isolates was screened for resistance to zinc and quaternary ammonium compounds (QACs). Multivariate analysis revealed that in-feed concentrations of zinc (P < 0.001) and frequent disinfection of nursery pens (P < 0.001) are associated with MRSA shedding in nursery pigs. Furthermore, 62.5% (25/40) of MRSA isolates carried the zinc resistance gene czrC and demonstrated decreased susceptibility to zinc. All MRSA isolates carried at least 1 QAC resistance gene. The most common genotype was qacG qacH smr, which occurred in 32.5% (13/40) of isolates. Seven isolates (17.5%) demonstrated a significant tolerance to benzalkonium chloride, indicating a potential to survive commercial QAC exposure in the presence of organic matter. Overall, these findings indicate that high levels of in-feed zinc and QAC-based disinfectants are important drivers in the selection and persistence of MRSA in commercial swine herds, and these agents may be coselecting for other antimicrobial resistance genes.     

Intestinal enterobacteria of the hibernating Apis mellifera mellifera L. bees

Dynamics of enterobacteria of normal intestinal microflora was studied in Apis mellifera mellifera L. bees hibernating under snow in the Western Urals. The cell numbers (N) of the predominant species Klebsiella oxytoca increased from 10-10(6) CFU/bee in November 2004 to 10(4)-10(7) CFU/bee in March 2005; its frequency of occurrence (P) increased from 92 to 100%. Increase of Providencia rettgeri (11.2004: N up to 10(6), P 25%; 03.2005: N 10(2)-10(6), P 80%) was accompanied by the substitution of Morganella morganii (11.2004: N up to 10(6), P 25%) with Proteus vulgaris (03.2005: N up to 10(5), P 8%). By spring, Hafnia alvei and Citrobacter sp., which are pathogenic to bees, disappeared (11.2004: N up to 10(5), P 13 and 10%, respectively). Endophytic species Pantoea agglomerans, Leclecria sp., and other representatives of the "Enterobacter agglomerans" group were present in November and after the first emergence in spring (N up to 10(5); November: P 15%; April: P 23%). In April, the number of enterobacteria decreased to 10(5), and P. rettgeri became the predominant species (P 54%) instead of K. oxytoca (P 43%).     

Enterotoxigenic capacity of strains of Klebsiella and Enterobacter genera isolated in acute intestinal diseases in children

234 strains, including 104 K. pneumoniae strains, 28 K. oxytoxica strains, 64 E. cloacae strains and 40 E. aerogenes strains, have been isolated from the intestine of 266 children with diarrhea, aged up to 1 year, and studied for enterotoxigenicity. By the coagglutination test, made with G. Kronvall's staphylococcal reagent prepared with the use of antiserum to Escherichia coli LT-enterotoxin, and the biological assay on suckling mice enterotoxigenic activity has been revealed in 119 strains, including 48 K. pneumoniae strains (12.6%), 33 E. cloacae strains (27.4%) and 23 E. aerogenes strains (19.7%). The strains producing only LT-enterotoxins, only ST-enterotoxins, and both LT- and ST-enterotoxins have been found. The determination of the enterotoxigenic activity of the clinical isolates of opportunistic enterobacteria makes it possible to improve the etiological interpretation of acute intestinal infections.     

Antimicrobial resistance patterns in Enterobacteriaceae isolated from an urban wastewater treatment plant

Over 18 months, enterobacteria were isolated from the raw (189 isolates) and treated (156 isolates) wastewater of a municipal treatment plant. The isolates were identified as members of the genera Escherichia (76%), Shigella (7%), Klebsiella (12%) and Acinetobacter (4%). Antimicrobial susceptibility phenotypes were determined using the agar diffusion method for the antibiotics amoxicillin, gentamicin, ciprofloxacin, sulfamethoxazole/trimethoprim, tetracycline and cephalothin, the disinfectants hydrogen peroxide, sodium hypochlorite, quaternary ammonium/formaldehyde and iodine, and the heavy metals nickel, cadmium, chromium, mercury and zinc. Class 1 integrons were detected by PCR amplification using the primers CS5 and CS3. Compared with the raw influent, the treated wastewater presented higher relative proportions of Escherichia spp. isolates resistant to ciprofloxacin and cephalothin (P<0.0001 and P<0.05, respectively). Except for mercury, which showed a positive correlation with tetracycline and sulfamethoxazole/trimethoprim, no significant positive correlations were observed between antibiotic, disinfectant and heavy metal resistance. The variable regions of class 1 integrons, detected in c. 10% of the Escherichia spp. isolates, contained predominantly the gene cassettes aadA1/dhfrI.     

Quinolone resistance in absence of selective pressure: the experience of a very remote community in the Amazon forest

Background

Quinolones are potent broad-spectrum bactericidal agents increasingly employed also in resource-limited countries. Resistance to quinolones is an increasing problem, known to be strongly associated with quinolone exposure. We report on the emergence of quinolone resistance in a very remote community in the Amazon forest, where quinolones have never been used and quinolone resistance was absent in 2002.

Methods

The community exhibited a considerable level of geographical isolation, limited contact with the exterior and minimal antibiotic use (not including quinolones). In December 2009, fecal carriage of antibiotic resistant Escherichia coli was investigated in 120 of the 140 inhabitants, and in 48 animals reared in the community. All fluoroquinolone-resistant isolates were genotyped and characterized for the mechanisms of plasmid- and chromosomal-mediated quinolone resistance.

Principal Findings

Despite the characteristics of the community remained substantially unchanged during the period 2002–2009, carriage of quinolone-resistant E. coli was found to be common in 2009 both in humans (45% nalidixic acid, 14% ciprofloxacin) and animals (54% nalidixic acid, 23% ciprofloxacin). Ciprofloxacin-resistant isolates of human and animal origin showed multidrug resistance phenotypes, a high level of genetic heterogeneity, and a combination of GyrA (Ser83Leu and Asp87Asn) and ParC (Ser80Ile) substitutions commonly observed in fluoroquinolone-resistant clinical isolates of E. coli.

Conclusions

Remoteness and absence of antibiotic selective pressure did not protect the community from the remarkable emergence of quinolone resistance in E. coli. Introduction of the resistant strains from antibiotic-exposed settings is the most likely source, while persistence and dissemination in the absence of quinolone exposure is likely mostly related with poor sanitation. Interventions aimed at reducing the spreading of resistant isolates (by improving sanitation and water/food safety) are urgently needed to preserve the efficacy of quinolones in resource-limited countries, as control strategies based only on antibiotic restriction policies are unlikely to succeed in those settings.     

Impact of an Urban Effluent on Antibiotic Resistance of Riverine Enterobacteriaceae and Aeromonas spp.

In order to evaluate the impact of an urban effluent on antibiotic resistance of freshwater bacterial populations, water samples were collected from the Arga river (Spain), upstream and downstream from the wastewater discharge of the city of Pamplona. Strains of Enterobacteriaceae (representative of the human and animal commensal flora) (110 isolates) and Aeromonas (typically waterborne bacteria) (118 isolates) were selected for antibiotic susceptibility testing. Most of the Aeromonas strains (72%) and many of the Enterobacteriaceae (20%) were resistant to nalidixic acid. Singly nalidixic acid-resistant strains were frequent regardless of the sampling site for Aeromonas, whereas they were more common upstream from the discharge for enterobacteria. The most common resistances to antibiotics other than quinolones were to tetracycline (24.3%) and beta-lactams (20.5%) for Enterobacteriaceae and to tetracycline (27.5%) and co-trimoxazole (26.6%) for Aeromonas. The rates of these antibiotic resistances increased downstream from the discharge at similar degrees for the two bacterial groups; it remained at high levels for enterobacteria but decreased along the 30-km study zone for Aeromonas. Genetic analysis of representative strains demonstrated that these resistances were mostly (enterobacteria) or exclusively (Aeromonas) chromosomally mediated. Moreover, a reference strain of Aeromonas caviae (CIP 7616) could not be transformed with conjugative R plasmids of enterobacteria. Thus, the urban effluent resulted in an increase of the rates of resistance to antibiotics other than quinolones in the riverine bacterial populations, despite limited genetic exchanges between enterobacteria and Aeromonas. Quinolone resistance probably was selected by heavy antibiotic discharges of unknown origin upstream from the urban effluent.