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ABSTRACTCampylobacter jejuniproduces a toxin called cytolethal distending toxin (CDT). Knowledge of the prevalence and homogeneity ofCampylobactersp. cdtgenes is incomplete. In this work, we identified four PCR primer pairs that collectively amplified cdtgenes in all of the C. jejuniandCampylobacter colistrains tested. Restriction analyses of the cdtPCR products showed clear differences between thecdtgenes of these two species, yet there were few heterogeneities noted between members of the same species. Consequently, it may be possible to speciate C. jejuniandC. coliisolates on the basis of restriction patterns within their cdtgenes.

Antibiotic resistance, incidence of multiple resistance and resistance patterns of fecal coliforms have been followed in a closed herd (nonantibiotic) in which all forms of antimicrobial agents have been discontinued for 154 months. Gradual, but fluctuating, decreases have been observed in antibiotic resistance and incidence of multiple resistance of fecal coliforms isolated from swine after antibiotic withdrawal. The percentage of fecal coliforms resistant to tetracycline has fluctuated between 20 and 44% since month 136. Age and type of housing of the pigs influenced the baseline level of antibiotic resistant fecal coliforms and incidence of multiple resistance. Results indicate that the selection of resistant fecal coliforms brought about by therapy or feed additive use may not be readily reversed by long-term withdrawal of antibiotics. The antibiotic resistant microflora that developed from the use of antibiotics prior to 1972 apparently has been passed on from one generation to the next even though antibiotics have not been used for any purpose in this herd. Comparisons also have been made of the antibiotic resistance of fecal coliforms from the nonantibiotic herd, a herd in which antibiotics are used for therapeutic or prophylactic purposes only and a herd continuously exposed to subtherapeutic levels of antibiotics. Percentage of antibiotic resistant fecal coliforms from the herd having occasional therapeutic treatment was higher than from the nonantibiotic herd and lower than from the continuously exposed herd. Therapeutic use of penicillin plus streptomycin resulted in increased percentages of fecal coliforms resistant to ampicillin, carbeniciliin and streptomycin. Tetracycline resistance of fecal isolates was approximately 26% for the nonantibiotic herd, 76% for the therapeutic only herd and 100% for the continuously exposed herd. When pigs from the nonantibiotic herd were moved, increases were observed in the population of antibiotic resistant fecal coliforms and in the incidence of multiple resistance, even though the pigs continued to be fed a nonantibiotic feed These resistance values remained elevated for long periods of time.

Chlortetracycline (CTC) resistance in anaerobic and coliform bacteria was examined in the large intestines of two groups of growing pigs that had received antibiotic-free diets since weaning. One group of pigs was from a low resistance herd (LR) that had not received antibiotics for 8 yr, while the other group of pigs was from a high resistance herd (HR) that routinely received antibiotics. After a 20-d adjustment period in a common production facility, LR pigs on an antibiotic-free diet had lower proportions of anaerobes (27%) and coliforms (22%) that were resistant to 25 μg CTC/ml than did similarly fed HR pigs (81 and 48%, respectively). Continued maintenance of LR and HR pigs on the antibiotic-free diet in a common production facility tended to increase resistance in anaerobes and coliforms from LR pigs to levels comparable with those in HR pigs at the end of an 85-d feeding trial, but not after 14 d. Administration of CTC in the feed at therapeutic (220 μg/g for 14 d) and growth-promoting (27.5 μg/g for 85 d) levels markedly increased percentages of resistant anaerobes and coliforms in LR pigs, but not in HR pigs after 14 d. Similar antibiotic effects were not seen at the end of the 85-d trial. These data demonstrate that, while exposure to antibiotics in feed may increase resistance in intestinal populations, other factors such as environment, cross-contamination and herd history can also influence the observed level of antibiotic resistance in swine.

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<.05) than that in isolates from AB pigs. The proportions of isolates resistant to ampicillin, kanamycin, neomycin, penicillin, streptomycin and sulfamethizole were lower (P<.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 µg/ g of diet) and at subtherapeutic (27.5 µg/g) levels for 14 d increased (P<.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.