Multiple antibiotic resistance in fecal, cecal and colonic coliforms from pigs fed therapeutic and subtherapeutic concentrations of chlortetracycline

Multiple resistance and antibiotic resistance were examined in coliforms isolated at slaughter from the large intestines of two groups of growing pigs that had received antibiotic-free diets since weaning. One group of pigs was from an antibiotic-fed (AB) herd that routinely received chlortetracycline (CTC) in feed, while the other group was from a nonantibiotic-fed (NAB) herd that had not received antibiotics for 8 yr. After a 20-d adjustment period in a common production facility, the mean number of antibiotics in the resistant pattern of isolates from NAB pigs was found to be lower (P less than .05) than that in isolates from AB pigs. The proportions of isolates resistant to ampicillin, kanamycin, neomycin, penicillin, streptomycin and sulfamethizole were lower (P less than .05) in NAB pigs than in AB pigs. Similar herd differences were not observed after pigs from both herds were maintained in a common production facility on an antibiotic-free diet for 105 d. Oral administration of CTC at therapeutic (220 micrograms/g of diet) and at subtherapeutic (27.5 micrograms/g) levels for 14 d increased (P less than .05) the mean numbers of antibiotics in the resistance patterns of isolates from NAB pigs, but did not alter multiple resistance in isolates from AB pigs. After 14 d, subtherapeutic CTC increased the percentage of isolates resistant to ampicillin, kanamycin, penicillin, streptomycin, tetracycline and sulfamethizole, while therapeutic CTC only increased the percentage of isolates from NAB pigs resistant to penicillin, tetracycline and sulfamethizole. Similar antibiotic effects were not seen in isolates from AB pigs after 14 d and were not seen in isolates from either group of pigs at the end of an 85-d feeding trial.     

Plasmids and resistance to antimicrobial agents and heavy metals in Staphylococcus hyicus from pigs and cattle

Staphylococcus hyicus-cultures, isolated from piglets and cattle with skin lesions were investigated for their plasmid content and their resistance to antimicrobial agents and heavy metals. Several plasmids of different sizes could be detected in most of the 32 "porcine" S. hyicus-isolates, whereas none of the 20 "bovine" S. hyicus-cultures possessed any plasmid. The "porcine" S. hyicus-isolates were much more resistant to antimicrobial substances than the "bovine" S. hyicus-cultures. However, the "porcine" and "bovine" S. hyicus-cultures did not differ in their resistance to heavy metals.     

Antimicrobial resistance of fecal Escherichia coli isolated from grow-finish pigs in 20 herds in Alberta and Saskatchewan.

Escherichia coli (n = 1439), isolated from the feces of apparently healthy grow-finish pigs in 20 herds in Alberta and Saskatchewan, were tested for susceptibility to 16 antimicrobials. All isolates were susceptible to amikacin, ceftriaxone, and ciprofloxacin and less than 1% was resistant to amoxicillin-clavulanic acid, cefoxitin, ceftiofur, gentamicin, and nalidixic acid. Resistance was most common to tetracycline (66.8%), sulfamethoxazole (46.0%) and streptomycin (33.4%). Twenty-one percent of the isolates were susceptible to all drugs, while 57% were resistant to 2 or more antimicrobials. Unconditional associations between resistances provided insight into the potential for co-selection. Every resistance-outcome was associated with at least 2 other drug-resistances. These associations illustrate the propensity for resistance phenotypes to occur together and the importance of considering co-selection in antimicrobial use decisions. A 2nd analysis explored the associations between resistance phenotypes in E. coli and Salmonella spp. from the same herd. Only 2 resistances in Salmonella were associated with herd-level E. coli resistance, indicating that E. coli is a poor sentinel for Salmonella AMR within herds. Herd-level management, including antimicrobial use, could affect antimicrobial resistance. The intra-class correlation between isolates within herds ranged from 0.1 to 0.46, which confirmed resistance clustered within herds. This suggests herd-level interventions might mitigate antimicrobial resistance. Overall, these results reflect the on-farm selection pressure for resistance and the potential food-safety risk from near-market animals. These data provide a baseline for comparisons with future on-farm monitoring of antimicrobial resistance in E. coli.     

Antimicrobial resistance in bacteria from horses: Epidemiology of antimicrobial resistance

Antimicrobial resistance poses a significant threat to the continued successful use of antimicrobial agents for the treatment of bacterial infections. While the epidemiology of antimicrobial resistance in bacteria from man has been studied extensively, less work has been undertaken in companion animals, particularly horses. Methicillin‐resistant Staphylococcus aureus has been identified as a cause of infections, with a low prevalence of nasal carriage by horses in the community but higher for hospitalised horses. Molecular characterisation has shown methicillin‐resistant Staphylococcus aureus strains either to be predominantly of types associated with horses or of sequence type ST398. Antimicrobial‐resistant Escherichia coli (including multidrug‐resistant and extended spectrum β‐lactamase‐producing isolates) have caused infections and been documented in faecal carriage by horses, with many significant resistance mechanisms identified. More sporadic reports and molecular characterisation exist for resistance in other bacteria such as enterococci, Salmonella, Acinetobacter and Pseudomonas species. Limited work has been undertaken evaluating risk factors and much of the epidemiology of antimicrobial resistance in bacteria from horses remains to be determined.     

Associations of antimicrobial uses with antimicrobial resistance of fecal Escherichia coli from pigs on 47 farrow-to-finish farms in Ontario and British Columbia

This study assessed the associations between antimicrobial use and other management practices in pigs and antimicrobial resistance in generic Escherichia coli recovered from feces of weaner and finisher pigs on 39 purposefully selected farrow-to-finish farms in Ontario and 8 in British Columbia. Antimicrobials (n = 13), most frequently penicillins and tetracycline, wer dministered to different age groups of pigs on study farms through various routes of administration. Logistic regression was used to model risk factors to antimicrobial resistance in fecal E. coli of pigs for the following antimicrobials: ampicillin, apramycin, carbadox, cephalothin, chloramphenicol, kanamycin, neomycin, nitrofurantoin, spectinomycin, streptomycin, sulfamethoxazole tetracycline, and cotrimoxazole (trimethoprim and sulfamethoxazole). Use of antimicrobials in weaner pigs compared with use in finisher pigs was associated with resistance in most models. There was phenotypic evidence of different mechanisms of resistance selection, including direct selection [use of carbadox was associated with carbadox resistance (OR = 6.48)]; cross-resistance [use of spectinomycin was associated with streptomycin resistance (OR = 2.29)]; and possible co-selection [ceftiofu use was associated with tetracycline resistance (OR = 6.12)]. These results provide further evidence that use of antimicrobials in pigs selects for resistance among fecal E. coli within and between classes of antimicrobials.     

Antibiotic resistance of fecal coliforms from swine fed subtherapeutic and therapeutic levels of chlortetracycline

The effect of feeding subtherapeutic (27.5 micrograms/g of diet for 85 d) and therapeutic (220 micrograms/g of diet for 14 d, followed by an antibiotic-free diet for 71 d) levels of chlortetracycline (CTC) on the antibiotic resistance of fecal coliforms of pigs from two herds (36 pigs/herd) with different histories of antibiotic exposure when housed in a newly constructed confinement facility was determined. The CTC-resistant coliforms were higher (65 vs 51%) for antibiotic (AB) pigs than for nonantibiotic (NAB) pigs after they had been fed an antibiotic-free diet for 21 d. Percentages of isolates resistant to ampicillin, kanamycin, neomycin and tetracycline and multiple antibiotic resistance were greater (P less than .05) in AB pigs after 21 d. Feeding subtherapeutic CTC resulted in a linear increase in CTC-resistant coliforms with time on experiment (P less than .03, NAB; P less than .06, AB). The CTC-resistant coliforms increased during the 14 d that therapeutic CTC was fed, then they decreased during the 71 d that the antibiotic-free diet was fed, resulting in a quadratic response with time (P less than .03, AB). Feeding subtherapeutic CTC resulted in a greater increase in CTC-resistant coliforms in AB (47%) than in NAB (23%) pigs. The CTC-resistant coliforms decreased after the therapeutic group had been returned to the antibiotic-free diet (P less than .05, NAB). Feeding CTC caused greater changes in the precentages of isolates from NAB pigs that were resistant to selected antibiotics and in multiple antibiotic resistance than in isolates from AB pigs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antimicrobial resistance in enteric pathogens isolated from Minnesota pigs from 1995 to 2004

This study investigated the occurrence and antimicrobial resistance profiles of Escherichia coli and Salmonella sp. isolated from swine samples submitted to the Minnesota Veterinary Diagnostic Laboratory (MVDL) in Saint Paul, Minnesota from 1995 to 2004. During this time period, a total of 5072 E. coli and 2793 Salmonella sp. was isolated. Most of these isolates were found to be resistant to the tetracycline and beta-lactam group of antibiotics. Resistance to spectinomycin was also frequently observed. An increasing trend in ampicillin resistance and a decreasing trend in apramycin resistance were seen in both pathogens, although ampicillin resistance was relatively higher in E. coli than in Salmonella. Aminoglycoside (amikacin) and quinolone (enrofloxacin) were the only antimicrobials to which minimum or no resistance was observed. The resistance of pig pathogens to several antibiotics indicates the need to routinely monitor the use of these antimicrobials and their associated resistance in pig populations.     

Antimicrobial resistance in fecal generic Escherichia coli and Salmonella spp. obtained from Ontario sheep flocks and associations between antimicrobial use and resistance

The purpose of this study was to examine the prevalence and patterns of antimicrobial resistance (AMR) in enteric bacteria obtained from Ontario sheep flocks, and associations between antimicrobial use (AMU) and AMR. Forty-nine sheep producers participated for a 1-year interval between 2006 and 2008. Two-hundred and eighty-three pooled fecal samples were collected from the flocks during initial and final visits. Up to 3 isolates of Salmonella spp. and generic E. coli per pooled fecal sample were tested for susceptibility to 15 antimicrobials. Resistance was infrequent among Salmonella (0%, n = 7 isolates) and low among E. coli (13.1%; n = 849) isolates. A small number of isolates were resistant to antimicrobials classified as being of very high importance to human health. Tetracycline resistance was most frequently observed (12.0%). Logistic regression was used to model potential AMU (qualitative and quantitative) risk factors for tetracycline resistance in generic E. coli from final visits. Qualitative analysis indicated that the use of injectable sulfonamides [including trimethoprim-sulfonamide combinations (TMS)] and tetracycline in the feed and water were significantly associated with tetracycline resistance (OR = 2.6, P = 0.01; and OR = 4.8, P ≤ 0.01, respectively). Quantitative analysis also indicated that TMS exposure rate was significantly associated with tetracycline resistance, which varied depending on the exposure rate. The exposure rate of tetracycline in the feed and water was only significant after the removal of one influential flock, warranting further research examining flocks with higher tetracycline exposure rates. Although the prevalence of AMR in participating flocks was relatively low, risk factors for resistance were identified.     

Antimicrobial resistance in generic fecal Escherichia coli from 29 beef farms in Ontario

The occurrence of antimicrobial resistance in generic Escherichia coli can serve as an indicator of the pool of resistance genes potentially available for transfer to pathogenic organisms. This study was conducted on 29 volunteer beef farms in Ontario to describe the prevalence and patterns of antimicrobial resistance in E. coli, and to describe changes in the prevalence of resistance during the feedlot stage of production. From the pooled fecal samples on 28 of the 29 farms, 31% of isolates from feedlots (n = 993) and 12% of isolates from cow-calf farms (n = 807) were resistant to one or more of 16 antimicrobials tested. No isolates were resistant to ceftriaxone, ciprofloxacin, gentamicin, or nalidixic acid, and < 1% of the pooled isolates were resistant to ceftiofur. Two percent of both feedlot and cow-calf isolates were resistant to > or = 5 antimicrobials. Cow-calf farms were at significantly lower risk than feedlots for having E. coli isolates that were resistant to streptomycin, sulfamethoxazole, and tetracycline. On average, the prevalence of sulfamethoxazole resistant E. coli isolates was significantly higher in calves than in cows. No resistance was observed to ceftriaxone or ciprofloxacin among isolates (n = 1265) obtained from individually sampled feedlot animals on 2 farms. Less than 1% of these isolates were resistant to gentamicin, nalidixic acid, and ceftiofur. From the individually sampled feedlot animals, resistance to streptomycin (on 1 farm), sulfamethoxazole, and tetracycline increased significantly from arrival to mid-point during the feeding period, and these levels persisted until market-readiness.     

Associations of antimicrobial use with antimicrobial resistance in Campylobacter coli from grow-finish pigs in Japan

To determine associations between antimicrobial use and antimicrobial resistance in Campylobacter coli, 155 isolates were obtained from the feces of apparently healthy grow-finish pigs in Japan. In addition, data on the use of antibiotics collected through the national antimicrobial resistance monitoring system in Japan were used for the analysis. Logistic regression was used to identify risk factors to antimicrobial resistance in C. coli in pigs for the following antimicrobials: ampicillin, dihydrostreptomycin, erythromycin, oxytetracycline, chloramphenicol, and enrofloxacin. The data suggested the involvement of several different mechanisms of resistance selection. The statistical relationships were suggestive of co-selection; use of macrolides was associated with enrofloxacin resistance (OR=2.94; CI(95%): 0.997, 8.68) and use of tetracyclines was associated with chloramphenicol resistance (OR=2.37; CI(95%): 1.08, 5.19). The statistical relationships were suggestive of cross-resistance: use of macrolides was associated with erythromycin resistance (OR=9.36; CI(95%): 2.96, 29.62) and the use of phenicols was associated with chloramphenicol resistance (OR=11.83; CI(95%): 1.41, 99.44). These data showed that the use of antimicrobials in pigs selects for resistance in C. coli within and between classes of antimicrobials.     

Antimicrobial and heavy metal resistance in commensal enterococci isolated from pigs

Antibiotic resistance in animal isolates of enterococci is of public health concern because of the risk of transfer of antibiotic resistance isolates or resistance determinants to consumers via the food chain. In this study, phenotypic and genotypic resistance in 192 pig isolates of enterococci to ampicillin, avilamycin, avoparcin, bacitracin, flavophospholipol, gentamicin, narasin, tetracycline, tiamulin, tylosin, vancomycin, virginiamycin, copper and zinc were investigated by susceptibility test and molecular methods. Resistance rates varied between the species but all isolates were susceptible to ampicillin, avilamycin, avoparcin, gentamicin and narasin but resistant to tetracycline and tylosin and intermediately resistant to copper. Only Enterococcus gallinarum and Enterococcus casseliflavus were resistant to vancomycin and virginiamycin resistance was present in less than half the Enterococcus faecium isolates. Zinc resistance was largely confined to Enterococcus faecalis but bacitracin resistance was uncommon in E. faecalis in comparison with the other species. Tiamulin resistance was common in all species except E. casseliflavus. Resistance to flavophospholipol was detected in most E. faecium isolates and in a high proportion of E. gallinarum, E. casseliflavus and E. hirae/durans but was only found in one isolate of E. faecalis. No tetO, rplC, rplD, vanA, vanB, vatA and vatD genes were found. The presence of ermB, tetL, tetM, tcrB, aac6-aph2, tetK, tetS, vanC1, vanC2, lsaA, lsaB and vatE varied between the species and largely corresponded to the susceptibility phenotype. The findings show that resistance to antibiotics of high clinical significance for nosocomial Enterococcus infections is absent, whereas antimicrobial resistance was detected for some other antibiotics including bacitracin, flavophospholipol, tetracycline, tiamulin, tylosin and virginiamycin.     

The resistance of antimicrobial agents in Salmonella from veterinary sources in Australia from 1975 to 1982

SUMMARY A survey of antibiotic resistance in 1,287 strains of Salmonella from bovine, porcine and avian sources in Australia was carried out from 1975 to 1982. Isolates were tested against ampicillin, chloramphenicol, furazolidone, neomycin, streptomycin and tetracycline. Resistance was found to streptomycin in 286 isolates and to tetracycline in 282 isolates. Resistance to other antimicrobials was low and was unrelated to source. One hundred and seventy-three isolates showed multiple resistance to 2 or more antimicrobial agents with resistance to streptomycin and tetracycline being the most common. The overall level of resistance did not change over the period examined.     

The sensitivity to antimicrobial agents of species of Yersinia isolated from cattle and pigs in Nigeria

Fourteen isolates of yersiniae belonging to three species isolated from cattle and pigs were tested for their sensitivity to 12 antimicrobial agents by the tube dilution technique. All the isolates were sensitive to gentamicin and 93.0%, 93.0%, 85.7% and 85.7% sensitive to tetracycline, nitrofurantoin, chloramphenicol and sulphamethoxazole-trimethoprim, respectively. There were no consistent differences in the rates and patterns of resistance based on source, species and serotypes of organisms.     

Antimicrobial resistance in Campylobacter spp. isolated from Ontario sheep flocks and associations between antimicrobial use and antimicrobial resistance

The objectives of this study were to determine the prevalence of antimicrobial resistance (AMR) in faecal Campylobacter spp. from lambs and adult sheep and associations between antimicrobial use (AMU) and AMR. A total of 275 faecal samples collected during initial and final visits from 51 sheep flocks, including one feedlot, across southern Ontario were tested for the presence of Campylobacter spp. Campylobacter jejuni was detected in 52% (143/275) of the faecal samples, Campylobacter coli in 7% (19/275), Campylobacter lari in 1% (2/275) and 2% (4/275) were non-speciated Campylobacter. Broth microdilution was used to test antimicrobial susceptibility of 162 isolates to nine antimicrobials. Campylobacter jejuni isolates (n = 142) were resistant to tetracycline (39%), ciprofloxacin (4%), nalidixic acid (4%) and telithromycin (1%). C. coli isolates (n = 19) were resistant to tetracycline (74%), and azithromycin, clindamycin, erythromycin, and telithromycin (5%). The C. lari isolate displayed resistance to nalidixic acid. No statistically significant associations were found between AMU and AMR during multivariate modelling in this study.     

Tetracycline-resistance in lactose-positive enteric coliforms originating from Belgian fattening pigs: degree of resistance, multiple resistance and risk factors

Between March and October 2003 a field study was conducted in 50 randomly selected pig herds to assess the degree of tetracycline-resistance in lactose-positive enteric coliforms (LPEC) originating from fattening pigs and to evaluate the combined effects of various husbandry conditions on the development and persistence of antimicrobial-resistant bacteria.

Data on housing, management and antimicrobial-drug consumption were collected, as well as faecal samples at three production stages: end of the nursery period (mean age: 72 days), end of the grower period (mean age: 125 days) and end of the finisher period (mean age: 186 days). The degree of tetracycline-resistant LPEC was determined by means of an agar dilution method.

Tetracycline-resistant LPEC were found in every herd. The overall degree of tetracycline-resistance in LPEC was 56.8% (S.D. 22.4%). Only a very weak relation was found between the degrees of TETR in the different production stages within the same herd, indicating that the degree of TETR is mainly associated with the production stage rather than with the farm as a whole.

The risk factor analysis showed that besides the antimicrobial-drug use, other factors like inside pen hygiene can influence the development and maintenance of antimicrobial-resistant bacteria in the gastrointestinal tracts of pigs. It was also observed that tetracycline-resistance in commensal Escherichia coli is often linked with resistance to other antimicrobial drugs like ampicillin and trimethoprim–sulphonamides.

These results illustrate that the epidemiology of antimicrobial resistance is influenced by antimicrobial-drug use, cross-resistance development and non-antimicrobial risk factors.     

Antimicrobial resistance in fecal Escherichia coli isolated from growing chickens on commercial broiler farms

To investigate the effects of rearing practices of commercial broiler chickens on the incidence of antimicrobial resistance in commensal Escherichia coli isolates, fecal E. coli isolates obtained in 4 farms were screened for anitimicrobial resistance. Ten E. coli isolates were recovered from each of the fecal samples collected from 10 birds in the farms at the ages of 2 days, 14-17 days, and 47-50 days. In 2 out of the 4 farms, no antimicrobials were used during the rearing period. In the other two farms, following collection of the fecal samples at 14 and 15 days of age, oxytetracycline (OTC), sulfadimethoxine (SDMX), and tylosin were given to birds on one farm and SDMX was used in the other. Isolates resistant to ampicillin and OTC that were obtained from an untreated flock at different sampling times were closely related to each other by pulsed-field gel electrophoresis patterns (PFGE) of XbaI-digested chromosomal DNA. PFGE analysis together with in vitro conjugation experiments suggested that diversity of resistance phenotypes within a clone may be resulted from the acquisition and loss of R-plasmids in an untreated and a treated flock. The numbers of resistance phenotypes observed among fecal isolates increased during the growth of the chickens in all the farms. The results in the present study suggest that persistence of commensal E. coli strains resistant to antimicrobials even in the absence of antimicrobial administration. It is also hypothesized that horizontal transmission of resistance determinants resulted in the emergence of different resistance phenotypes in those farms.     

Trends in the resistance to antimicrobial agents of Streptococcus suis isolates from Denmark and Sweden

This study was conducted to determine the MIC values of historical and contemporary Streptoccocus suis (serotypes 2 and 7) from Denmark and S. suis (serotype 2) from Sweden. A total of 52 isolates originating from 1967 through 1981 and 156 isolates from 1992 through 1997 in Denmark and 13 isolates from Sweden were examined for their MICs against 20 different antimicrobial agents. Most antimicrobials were active against most isolates. A frequent occurrence of resistance to sulphamethoxazole was observed, with most resistance among historic isolates of serotype 7 and least resistance among isolates from Sweden. A large number of the isolates was resistant to macrolides. However, all historic serotype 2 isolates from Denmark were susceptible, whereas 20.4% of the contemporary isolates were resistant. Among serotype 7 isolates 23.3% of the historic isolates were resistant to macrolides, whereas resistance was found in 44.8% of the contemporary isolates. All isolates from Sweden were susceptible to macrolides. Time-associated frequency of resistance to tetracycline was also found. Only a single historic isolate of serotype 2 was resistant to tetracycline, whereas 43.9% of the contemporary serotype 2 isolates and 15.5% of the contemporary serotype 7 isolates were resistant. Only one (7.7%) of the isolates from Sweden was resistant. The differences in resistance between historic and contemporary isolates from Denmark were statistically significant. This study demonstrated a significant serotype-associated difference in the susceptibility to macrolides and tetracycline and demonstrated that an increase in resistance among S. suis isolates has taken place during the last 15 years to the two most commonly used antimicrobial agents (tylosin and tetracycline) in pig production in Denmark.