Cross-resistance and associated resistance in 2478 Escherichia coli isolates from the Pan-European ECO.SENS Project surveying the antimicrobial susceptibility of pathogens from uncomplicated urinary tract infections

The antimicrobial resistance profiles, comprising 12 antibiotics, of 2478 isolates of Escherichia coli from the ECO.SENS Project involving women with acute uncomplicated urinary tract infection at 252 community health care centres in 17 countries were determined. Resistance to ampicillin alone (6.3%) and sulfamethoxazole alone (5.4%) were the most common 'single resistances'. Multiple resistance was most common in Spain and least common in Finland. The main associated-resistance profiles involved ampicillin/sulfamethoxazole (8.7%) and ampicillin/sulfamethoxazole/trimethoprim/trimethoprim-sulfamethoxazole (6.4%). The most common profile of multiple resistance was ampicillin/sulfamethoxazole/trimethoprim/trimethoprim-sulfamethoxazole/nalidixic acid/ciprofloxacin. Twenty-one isolates, half of which came from Spain, were resistant to seven antibiotics or more. Three isolates, one from Spain and two from Portugal, were resistant to nine of the 12 antibiotics investigated.     

Non-hospital antimicrobial usage and resistance in community-acquired Escherichia coli urinary tract infection

OBJECTIVES: To investigate the correlation between non-hospital antimicrobial consumption and resistance. METHODS: Information on the non-hospital sales of antimicrobials from 14 European countries in 1997 and 2000 was compared with the antimicrobial resistance profiles of Escherichia coli isolated from women with community-acquired urinary tract infection in the same countries in 1999/2000. RESULTS: There was no statistically significant correlation between the consumption of and resistance to co-amoxiclav, cefadroxil, fosfomycin, mecillinam, sulfamethoxazole, trimethoprim or trimethoprim-sulfamethoxazole. On the other hand, there were statistically significant correlations between consumption of broad-spectrum penicillins and quinolones in 1997 and 2000 and resistance to ciprofloxacin (P range 0.0005-0.0045) and nalidixic acid (P range 0.0013-0.0049). Total antimicrobial consumption in 1997 was significantly correlated to ciprofloxacin (P=0.0009) and nalidixic acid (P=0.0018) resistance, and there were significant relationships between quinolone consumption in both years and resistance to gentamicin (P range 0.0029-0.0043) and nitrofurantoin (P range 0.0003-0.0007). E. coli with multiple antimicrobial resistance were significantly more common in countries with high total antimicrobial consumption. CONCLUSIONS: Owing to the frequent presence of many possible confounding factors, antimicrobial resistance to one drug does not always correlate well to the consumption of the same drug or closely related drugs. This study showed that the degree of antimicrobial consumption was significantly correlated to the incidence of multidrug-resistant E. coli.     

High prevalence of antibiotic resistant Escherichia coli in faecal samples of students in the south-east of The Netherlands

From December 1988 to March 1989 172 faecal samples from first and second year students at the University of Limburg, Maastricht, The Netherlands, were collected and analysed for the presence of Escherichia coli strains resistant to ampicillin, sulphamethoxazole, tetracycline, trimethoprim and nitrofurantoin. In addition, the antibiotic susceptibility (MIC) to these agents and six other compounds (i.e. aminoglycosides, nitrofurantoin, nalidixic acid and cephalothin) was determined. The prevalence of resistance of the samples analysed ranged from 86% (i.e. 148 out of 172) for sulphamethoxazole to 25% for trimethoprim. The prevalence figures for ampicillin, tetracycline and nitrofurantoin resistance were 76%, 47% and 29%, respectively. The percentages of the faecal samples with a high proportion of E. coli (greater than 50% of the total number of E. coli) resistant to ampicillin, tetracycline and sulphamethoxazole were 8%, 11% and 37%, respectively. For nitrofurantoin and trimethoprim the figures were 3% and 1%, respectively. Of the isolated E. coli (n = 797), 84% was resistant to ampicillin, 82% resistant to sulphamethoxazole. The lowest percentages (9%) were observed for both nalidixic acid and nitrofurantoin.     

The ECO.SENS Project: a prospective, multinational, multicentre epidemiological survey of the prevalence and antimicrobial susceptibility of urinary tract pathogens--interim report

The ECO.SENS Project is the first international survey to investigate the prevalence and susceptibility of pathogens causing community-acquired, uncomplicated urinary tract infections (UTIs) in women. At 240 centres in 17 countries, female patients presenting with symptoms of uncomplicated UTIs were asked to provide a urine sample for testing for the presence of leucocytes and bacteria. The bacteria were identified and their susceptibility to 12 antibiotics commonly used in the treatment of UTIs was determined. The objective of the survey was to collect 5000 urine samples to obtain approximately 3500 isolates of defined uropathogens. This interim report includes the results from 1960 urine samples, 75% of which contained a uropathogen. Escherichia coli accounted for the majority (80%) of uropathogens isolated in all 17 countries. The rates of resistance among E. coli strains were: ampicillin and sulphamethoxazole, 30%; trimethoprim alone or with sulphamethoxazole, 15%; nalidixic acid, 6%; ciprofloxacin, 3%; amoxycillin-clavulanic acid, mecillinam, cefadroxil, nitrofurantoin and fosfomycin, < or =2%. The use of ampicillin, sulphonamides and trimethoprim alone or with sulphamethoxazole needs to be reconsidered. The seemingly rapid increase in quinolone resistance among community-acquired E. coli in some of the countries gives cause for concern.     

Risk factors for antibiotic-resistant Escherichia coli isolated from community-acquired urinary tract infections in Dakar, Senegal

OBJECTIVES: To assess overall resistance rates and risk factors for resistance to ampicillin, co-amoxiclav, nalidixic acid, fluoroquinolones and trimethoprim/sulfamethoxazole in Escherichia coli strains isolated from outpatients with acute urinary tract infection in Dakar (Senegal). PATIENTS AND METHODS: From June 2001 to June 2003, a prospective study was performed among Senegalese outpatients consulting at the Institut Pasteur of Dakar for urine analysis. Evaluated risk factors were: age, gender, prior hospitalization, antibiotic exposure, urinary tract infection and urinary catheter. RESULTS: A total of 398 non-duplicate, consecutive, biologically significant E. coli were isolated. The levels of antibiotic resistance in Dakar appeared dramatic and worrisome with resistance rates ranging from 18.6% for fluoroquinolones to 73.6% for ampicillin. With the exception of the presence of urinary catheter, the risk factors identified were consistent with data previously reported in developed countries. CONCLUSIONS: We hope our results will assist medical authorities in the development of appropriate control strategies.     

A European survey of antimicrobial susceptibility among zoonotic and commensal bacteria isolated from food-producing animals

OBJECTIVE: To study antimicrobial resistance in zoonotic bacteria isolated from food animals in different countries using uniform methodology. METHODS: Samples were taken at slaughter from chickens, pigs and cattle in four EU countries per host. Escherichia coli (indicator organism; n = 2118), Salmonella spp. (n = 271) and Campylobacter spp. (n = 1325) were isolated in national laboratories and MICs tested in a central laboratory against, where appropriate, ampicillin, cefepime, cefotaxime, ciprofloxacin, chloramphenicol, erythromycin, gentamicin, nalidixic acid, streptomycin, tetracycline and trimethoprim/sulfamethoxazole. RESULTS: Isolation rates were high for E. coli, low for Salmonella and intermediate for Campylobacter. MIC results showed resistance prevalence varied among compounds, hosts and countries. For E. coli and Salmonella, resistance to newer compounds (cefepime, cefotaxime, ciprofloxacin) was absent or low, but to older compounds (except gentamicin), resistance was variable and higher. E. coli isolates from Sweden showed low resistance, whereas among isolates from Spain (pigs), resistance to ampicillin, chloramphenicol, streptomycin, tetracycline and trimethoprim/sulfamethoxazole was higher; the UK, France, the Netherlands, Germany, Italy and Denmark were intermediate. For Campylobacter spp. isolates from chickens, nalidixic acid and ciprofloxacin resistance was >30% in France and the Netherlands, >6% in the UK and zero in Sweden. Nalidixic acid resistance was high in cattle (20%-64%), whereas ciprofloxacin resistance was markedly lower in cattle, variable in pigs (3%-21%) and highest in Sweden. Generally, Campylobacter coli was more resistant than Campylobacter jejuni. CONCLUSION: Antimicrobial resistance among enteric organisms in food animals varied among countries, particularly for older antimicrobials, but resistance to newer compounds used to treat disease in humans was generally low.     

Antibiotic resistance and small R plasmids among Escherichia coli isolates from outpatient urinary tract infections in northern Norway.

Escherichia coli strains from outpatient urinary tract infections in northern Norway over a period of 1 year were examined for resistance to nine commonly used antibiotics. Strains collected during 4.5 months were examined for R plasmid content by using conjugation and in vitro transformation. Of the E. coli strains, 42% were resistant to one or more antibiotics. Resistance was highest to sulfonamide (20.8% of all strains), nitrofurantoin (14.5%), and tetracycline (10.1%), whereas less than 6% of the strains were resistant to ampicillin, carbenicillin, cephalothin, nalidixic acid, or trimethoprim-sulfamethoxazole. No strain was resistant to gentamicin. Tetracycline resistance was more common in men than in women. Resistance to cephalothin, nalidixic acid, and sulfonamide was higher in strains from older people. Resistance to sulfonamide was more frequent in the urban community. These was no seasonal variation in antibiotic resistance, although the incidence of urinary tract infection varied with seasons. Plasmid-determined resistance to ampicillin, streptomycin, sulfonamide, and tetracycline was found. About 18% of the resistant strains from the urban municipality carried R plasmids, most of which were small plasmids mediating resistance to sulfonamide and streptomycin. The overall frequency of resistance in strains collected from rural areas was similar to the urban frequency, but in the rural strains, R plasmids were found in only 5% of the resistant strains.     

Phenotypic and genotypic characterization of antimicrobial resistance in Escherichia coli O111 isolates

OBJECTIVES: The aim of this study was to generate baseline data on the prevalence and molecular basis of antimicrobial resistance in Escherichia coli O111 isolates. METHODS: A total of 105 epidemiologically unrelated E. coli O111 isolates from humans and cattle (isolated between 1983 and 2003) were tested for susceptibility to 17 antimicrobial agents by broth microdilution. Resistant isolates were screened by molecular methods for resistance genes, class 1 and 2 integrons and mutations in the quinolone-resistance determining regions. RESULTS: Resistance was found in 76% of the isolates, with a prevalence of 72% for multiresistance. The most prevalent resistances were to streptomycin, sulfamethoxazole and tetracycline (72-68%), followed by spectinomycin, ampicillin and kanamycin/neomycin (39-25%). For each antimicrobial agent, the predominant resistance genes were ampicillin, bla(TEM) (94%); chloramphenicol, catA1 (100%); gentamicin, aac(3)-IV and aac(3)-II (50% each); kanamycin, aphA1 (100%); streptomycin, aadA1- like (66%); sulfamethoxazole, sul1 (59%); tetracycline, tet(A) (86%); and trimethoprim, dfrA1-like (83%). Class 1 integrons were found in 41% of the isolates. They carried aadA1, dfrA1-aadA1 and dfrA15-aadA1. A class 2 integron (dfrA1-sat1-aadA1) was found in one isolate. Only three isolates (3%) were resistant to nalidixic acid (reduced susceptibility to ciprofloxacin), with a single mutation in the gyrA gene. CONCLUSIONS: E. coli O111 strains exhibit a wide repertoire of genetic elements to sustain antimicrobial pressure. Two specific antimicrobial resistance pheno/genotypes, [STR-SPT]-SUL-TET/aadA1-sul1-tet(A) and STR-SUL-TET-AMP-[KAN-NEO]/strA/B-sul2-tet(A)-bla(TEM)-aphA1, are predominant.     

The status of antimicrobial resistance in Taiwan among gram-negative pathogens: the Taiwan surveillance of antimicrobial resistance (TSAR) program, 2000

In a nationwide surveillance of antimicrobial resistance (Taiwan Surveillance of Antimicrobial Resistance, TSAR), isolates were collected from 21 medical centers and regional hospitals throughout Taiwan over a three-month period in 2000 (TSAR II). This report summarizes susceptibility data of 7 common Gram-negative bacilli (Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Proteus mirabilis, Enterobacter cloacae, Pseudomonas aeruginosa, and Acinetobacter baumannii) in the TSAR II collection and compared selected key forms of resistance by epidemiologic factors and with isolates collected in 1998 (TSAR I) as well as with data from international surveillance studies. Resistance of the 5 Enterobacteriaceae species to most of the commonly prescribed "first-line" antimicrobials in Taiwan, such as ampicillin (78% in E. coli, 68% in P. mirabilis), gentamicin (19% in K. pneumonia to 66% in S. marcescens), and trimethoprim/sulfamethoxazole (29% in K. pneumoniae to 70% in P. mirabilis), was high, several of which are higher than other countries. Resistance to certain broad-spectrum antimicrobials is also more acute in Taiwan than most Western countries, such as ceftazidime resistant A. baumannii (73%) and ciprofloxacin resistant E. coli (12%). Differences in geographic regions and specimen types were associated with certain forms of resistance in TSAR II; however, the resistance problem is prevalent among both inpatients and outpatients of not only medical centers but also regional hospitals throughout Taiwan.     

Association between antibiotic resistance and the expression of Dr adhesin among uropathogenic Escherichia coli

BACKGROUND: Urinary tract infections (UTIs) are a significant health problem and Escherichia coli has been exported to be the primary pathogen in approximately 80% of cases. E. coli express structures called adhesins, fimbriae or pili that help them bind to specific tissue receptors. One such adhesin is Dr, which binds to the complement decay-accelerating factor (CD55) on the host cell. The purpose of the present study was to review the epidemiology and antimicrobial sensitivity spectrum of Dr adhesin-bearing E. coli isolates in women with UTI. METHODS AND RESULTS: A total of 337 uropathogenic E. coli isolates were collected, and mannose-resistant hemagglutination was observed in 43%, while 12.4% expressed Dr adhesin. All 337 uropathogenic E. coli isolates were found to be resistant to penicillin, oxacillin, bactericin and cloxacillin. None of the isolates was resistant to quinolones. The resistance to sulfamethoxazole was most common (90%), followed by nalidixic acid (51%) and ampicillin (41%). Interestingly, uropathogenic E. coli expressing Dr adhesin had a high incidence of ampicillin resistance (83%), and only 17% of the Dr-bearing isolates were found to be resistant to an ampicillin/sulbactum combination. CONCLUSION: These results indicate that UTIs may be successfully treated with ampicillin in combination with a beta-lactamase inhibitor, so that selective colonization by Dr adhesin-bearing uropathogenic E. coli is prevented, which is probably promoted by treating these patients with ampicillin alone.     

Quinolone, fluoroquinolone and trimethoprim/sulfamethoxazole resistance in relation to virulence determinants and phylogenetic background among uropathogenic Escherichia coli

INTRODUCTION: The goal of this study was to assess how resistance to quinolones, fluoroquinolones and trimethoprim/sulfamethoxazole relates to the virulence potential and phylogenetic background of clinical Escherichia coli isolates. METHODS: Among 150 uropathogens (21% resistant to quinolones, 12% resistant to fluoroquinolones and 29.3% resistant to trimethoprim/sulfamethoxazole), E. coli phylogenetic group, 15 virulence-associated genes and 7 O antigens were analysed. Clonal group A (CGA) and genomic PCR profiles were studied among trimethoprim/sulfamethoxazole-resistant isolates. RESULTS: Isolates susceptible to the three antimicrobial agents were significantly associated with phylogenetic group B2, whereas resistant isolates exhibited shifts to non-B2 groups (quinolone and fluoroquinolone-resistant isolates to group A; trimethoprim/sulfamethoxazole-resistant isolates to group D). Diverse virulence traits, including UTI-associated O antigens, were significantly less frequent among resistant isolates, particularly those resistant to fluoroquinolones (median score, 3.9 virulence factors/strain) and also to quinolones (5.2) or trimethoprim/sulfamethoxazole (6.4), as compared with the corresponding drug-susceptible isolates (median scores of 7.9, 8.6 and 7.9, respectively). Among 44 trimethoprim/sulfamethoxazole-resistant isolates, 3 (6.8%) belonged to CGA. All these 3 CGA strains caused pyelonephritis (P=0.02) and exhibited the consensus virulence profile of previously described CGA strains from abroad. CONCLUSIONS: E. coli isolates resistant to quinolones, trimethoprim/sulfamethoxazole and especially fluoroquinolones were associated with reductions in virulence traits and shifts to non-B2 phylogenetic groups. Moreover, fluoroquinolone resistance usually occurred in low-virulence E. coli group A isolates rather than in isolates from groups B2 and D which had lost virulence traits. CGA accounted for 23% of trimethoprim/sulfamethoxazole-resistant E. coli producing pyelonephritis.     

Susceptibility of shigellae to mecillinam, nalidixic acid, trimethoprim, and five other antimicrobial agents.

A total of 199 strains of shigella (1 Shigella dysenteriae, 15 S. boydii, 47 S. flexneri, and 136 S. sonnei) isolated in Malmö, Sweden, within a 3-year period (1977 through January 1980) were tested with the agar plate dilution method for susceptibility to commonly used and newer antimicrobial agents. Mecillinam, nalidixic acid, and trimethoprim had the best in vitro activity. S. flexneri dominated among strains resistant to three or more antimicrobial agents and were less susceptible to ampicillin, chloramphenicol, and doxycycline than other types studied. Sixty-four percent of the strains were resistant to sulfamethoxazole. In vitro, a synergistic effect with trimethoprim was shown only in strains susceptible to sulfamethoxazole. The amidinopenicillin mecillinam was highly active against shigellae. When resistance occurred, it was linked to ampicillin in 17 of 18 strains. The quinolines, here represented by nalidixic acid, might be the drugs of choice.     

Acquisition and epidemiology of antibiotic-resistant Escherichia coli in a cohort of newborn calves

OBJECTIVES: The acquisition of antibiotic-resistant commensal Escherichia coli was examined in a cohort of newborn calves. METHODS: Faecal samples were collected weekly from calves over a 4 month period and screened for E. coli resistant to ampicillin, apramycin and nalidixic acid at concentrations of 16, 8 and 8 mg/L, respectively. E. coli viable counts were performed on samples from a subset of calves. RESULTS: All calves acquired ampicillin- and nalidixic acid-resistant E. coli, while only 67% acquired apramycin-resistant E. coli during the study. Sixty-seven per cent of samples were resistant to at least one of the three antibiotics. Prevalence of ampicillin and nalidixic acid resistance was high initially and declined significantly with age (P < 0.001). No temporal or age-related pattern was observed in the prevalence of apramycin resistance. Housing the cohort had a significant effect on the prevalence of nalidixic acid resistance (P < 0.001). Total and ampicillin- and nalidixic acid-resistant E. coli counts declined with calf age (P < 0.001), with the rate of decline in ampicillin-resistant counts being greater than that for total counts (P < 0.001). The proportion of total E. coli counts that were resistant to ampicillin or nalidixic acid also declined with age (P < 0.001). CONCLUSIONS: Cohort calves rapidly acquired antibiotic-resistant bacteria within days of birth. Carriage of resistant bacteria was associated with both age and housing status of the cohort.     

Antibiotic resistance in salmonellae isolated from humans and animals in France: comparative data from 1994 and 1997

Among 25526 recorded isolates of salmonellae, 5086 isolated from humans and 20440 from animals in 1994 and 1997 in France, the antibiotic resistance phenotype was determined for all human and 5336 animal isolates. In Salmonella enterica serovar typhimurium, one of the two most frequently isolated serovars from humans as well as animals, resistance to ampicillin was observed in 61% of both human and animal isolates in 1994 and in 73% of human and 53% of animal isolates in 1997. During these periods, resistance to co-amoxiclav was between 45% and 66% for both types of isolate. Resistance to ampicillin was associated with resistance to streptomycin, spectinomycin, sulphonamide, tetracycline and chloramphenicol in over 70% of isolates. Resistance to ampicillin as well as co-amoxiclav never exceeded 7% in Salmonella enteritidis. While Salmonella hadar was practically absent among the human isolates in 1994, this serovar was the third most frequent in 1997, and at that time 92% were resistant to nalidixic acid. Among the animal S. hadar isolates, the prevalence of resistance to nalidixic acid increased from 3% in 1994 to 72% in 1997. None of these isolates manifested high-level resistance to ofloxacin. The levels of resistance to aminoglycosides (< or =3%) and trimethoprim-suphamethoxazole (< or =14%) remained practically unchanged in all three serovars. The resistance markers of 463 ampicillin-resistant S. typhimurium isolated in 1997 were determined. Among the 24 phenotypes observed, six multiresistance phenotypes, representing 82% of these isolates (as compared with 80% in 1994), were associated with the PSE-1 gene typically found in the lysotype DT104 of this serovar.     

Phenotypic and genotypic characterization of antimicrobial resistance in German Escherichia coli isolates from cattle,swine and poultry

OBJECTIVE: Phenotypic and genotypic characterization of the antimicrobial resistance of German Escherichia coli strains isolated during 1999-2001 from cattle, swine and poultry. MATERIALS AND METHODS: Three hundred and seventeen isolates were tested for their resistance to 17 antimicrobial agents by broth microdilution. Resistant strains were screened by molecular methods for resistance genes, integrons and mutations in quinolone-resistance determining regions. RESULTS: Resistance was found in 40% and multiresistance in 32% of the strains. The resistance was significantly higher in isolates from poultry (61%) and swine (60%) than from cattle (25%) (P < 0.01). The most prevalent resistances were to sulfamethoxazole, tetracycline, streptomycin, ampicillin and spectinomycin (30-15%). For each antibiotic, the predominant resistance genes were: ampicillin, blaTEM1-like (92%); chloramphenicol, catA (68%) and cmlA1-like (36%); gentamicin, aac(3)-IV (60%); kanamycin, aphA1 (100%); streptomycin, aadA1-like (61%) and strA/B (59%); sulfamethoxazole, sul2 (66%), sul1 (42%) and sul3 (14%); tetracycline, tet(A) (66%) and tet(B) (42%); and trimethoprim, dfrA1-like (77%), dfrA17 (13%) and dfrA12 (7%). Class 1 integrons were found in 30% of the strains. They carried dfrA1-aadA1a (40%), aadA1a (29%), sat1-aadA1a (16%), dfrA17-aadA5 (11%), oxa1-aadA1a (5%) and dfrA12-aadA2 (3%). Eleven percent of the strains were resistant to nalidixic acid. Of these, 61% presented a reduced susceptibility to ciprofloxacin (MIC = 0.12-2 mg/L) and single mutations in gyrA or gyrA and parC genes, and 39%, full resistance to ciprofloxacin (MIC > or = 4 mg/L) and double and single mutations in gyrA and parC, respectively. CONCLUSION: The study gives baseline information on the magnitude of the resistance problem and its genetic background in contemporary German E. coli from food-producing animals.     

Emergence of multidrug-resistant Salmonella enterica serovar Typhi with reduced susceptibility to fluoroquinolones in Cambodia

From December 2006 to April 2009, we conducted an etiology study among Cambodian patients presenting with acute fever of unknown origin. Salmonella enterica serovar Typhi was detected in 0.9% (41/4985) blood cultures. Antimicrobial susceptibility testing showed decreased susceptibility to ampicillin (56% resistant; MIC₉₀, >256 μg/mL), chloramphenicol (56% resistant; MIC₉₀, >256 μg/mL), trimethoprim/sulfamethoxazole (56% resistant; MIC₉₀, >256 μg/mL), nalidixic acid (81% resistant; MIC₉₀, not defined), ciprofloxacin (0% resistant; MIC₉₀, 0.5 μg/mL), and ceftriaxone (0% resistant; MIC₉₀, 0.094 μg/mL). Multidrug resistance, defined as antimicrobial resistance to ampicillin, chloramphenicol, and trimethoprim/sulfamethoxazole, was found in 56% of the isolates, and 80% had reduced susceptibility to ciprofloxacin (defined as MIC ≥0.12 μg/mL).     

Emergence of aminoglycoside 3-N-acetyltransferase IV in Escherichia coli and Salmonella typhimurium isolated from animals in France.

We studied two outbreaks of calf salmonellosis caused by apramycin and gentamicin-resistant Salmonella typhimurium strains. In both cases, the responsible strains were resistant to ampicillin, chloramphenicol, kanamycin, streptomycin, tetracycline, and trimethoprim; one strain was also resistant to nalidixic acid in one outbreak. A systematic survey of the intestinal Escherichia coli strains of calves from the two affected flocks showed that 11 of 24 animals sampled were also colonized by apramycin- and gentamicin-resistant E. coli strains. These isolates belonged to four biotypes and were resistant to ampicillin, chloramphenicol, kanamycin, streptomycin, tetracycline, trimethoprim, and nalidixic acid. All of the strains were resistant to high levels of apramycin (MICs, 512 to 1,024 micrograms/ml) and to gentamicin (MICs, 8 to 32 micrograms/ml), and these resistances were always transferred en bloc. In S. typhimurium, this coresistance was borne by plasmids that were approximately 39 kilobases long (outbreak 1) or 90 kilobases long (outbreak 2), whereas in E. coli, the coresistance was due to plasmids that were approximately 110 kilobases long in both outbreaks. The two plasmids of Salmonella and four plasmids of E. coli encoded type IV aminoglycoside 3-N-acetyltransferases. The intensive use of curative and preventive treatments in calf production could be responsible for the emergence of enzymic resistance to apramycin and gentamicin.     

Bactericidal activity of trimethoprim alone and in combination with sulfamethoxazole on susceptible and resistant Escherichia coli K-12.

The combined effects of trimethoprim and sulfamethoxazole on the viability of Escherichia coli K-12 and resistant strains possessing resistance plasmids were examined in minimal medium. When methionine, glycine, and adenine were present, sulfamethoxazole could enhance trimethoprim activity against E. coli K-12 so that the combination was bactericidal. However, this enhancement occurred over a narrow range of trimethoprim concentrations (0.04 to 0.2 mg liter-1) and only when the sulfamethoxazole concentration was more than 10 times that of trimethoprim. Under certain conditions, sulfamethoxazole enhanced trimethoprim bactericidal activity against E. coli K-12 carrying plasmid R1 at concentrations of sulfamethoxazole far below those required to inhibit the organism, but there was no such enhancement with the same host containing the SSu plasmid. Similar differences were found with strains possessing trimethoprim resistance plasmids R483 and R751. Sulfamethoxazole can promote a bactericidal response with trimethoprim in E. coli K-12 and some of its resistant derivatives, but only under a narrow range of concentrations.     

Genetic relatedness of a rarely isolated Salmonella: Salmonella enterica serotype Niakhar from NARMS animal isolates

BACKGROUND: In the United States, Salmonella enterica serotype Niakhar is infrequently isolated. Between 1997 and 2000, the animal arm of the National Antimicrobial Resistance Monitoring System-Enteric Bacteria (NARMS) assayed a total of 22,383 Salmonella isolates from various animal sources (swine, cattle, chickens, turkeys, cats, horses, exotics and dogs) for antimicrobial susceptibility. Isolates originated from diagnostic and non-diagnostic submissions. OBJECTIVES: To study the phenotypic and genotypic characteristics of Salmonella Niakhar. METHODS AND RESULTS: Only five (0.02%) of the 22,383 isolates were identified as Salmonella Niakhar. Antimicrobial resistance testing indicated that three isolates were pan-susceptible, one isolate was resistant to ampicillin and one isolate was resistant to ampicillin, chloramphenicol, ciprofloxacin, kanamycin, nalidixic acid, streptomycin, sulfamethoxazole, tetracycline and trimethoprim/sulfamethoxazole. RAPD-PCR analysis, PFGE and ribotyping indicated that two pan-susceptible isolates were genetically similar, whereas the three remaining isolates were genetically different. The one Salmonella Niakhar isolate that was multiresistant harboured a class I integron, intI1 and two large plasmids. CONCLUSIONS: This study represents the first report of a ciprofloxacin-resistant Salmonella isolate from the animal arm of NARMS.     

Identification of antimicrobial resistance and class 1 integrons in Shiga toxin-producing Escherichia coli recovered from humans and food animals

OBJECTIVES: The objective of this study was to identify antimicrobial resistance and class 1 integrons among Shiga toxin-producing Escherichia coli (STEC). METHODS: Two-hundred and seventy-four STEC recovered from poultry, cattle, swine and humans were characterized by antimicrobial susceptibility testing, screened for the presence of class 1 integrons by PCR, and assayed for integron transfer by conjugation. RESULTS: Ninety-three (34%) of the isolates were resistant to streptomycin, followed by 89 (32%) to sulfamethoxazole, 83 (30%) to tetracycline, 48 (18%) to ampicillin, 29 (11%) to cefalothin, 22 (8%) to trimethoprim/sulfamethoxazole, 18 (7%) to gentamicin, 13 (5%) to chloramphenicol and 10 (4%) to cefoxitin. Class 1 integrons were detected in 43 (16%) of the 274 isolates. The adenyl acetyltransferase gene, aadA, which confers resistance to streptomycin, was identified in integrons from 41 (95%) of these 43 isolates, and the dfrA12 gene, which confers resistance to trimethoprim, was identified in integrons from eight (19%) of the isolates. The sat1 gene, which confers resistance to streptothricin, an antimicrobial that has never been approved for use in the United States, was identified in integrons from three (7%) of the isolates. Transfer of integrons by conjugation between strains of E. coli resulted in transfer of antimicrobial-resistant phenotypes for ampicillin, chloramphenicol, cefalothin, gentamicin, tetracycline, trimethoprim, sulfamethoxazole and streptomycin. CONCLUSIONS: Antimicrobial resistance is common in STEC. Class 1 integrons located on mobile plasmids have facilitated the emergence and dissemination of antimicrobial resistance among STEC in humans and food animals.