Antibiotic resistance in anaerobic and coliform bacteria from the intestinal tract of swine fed therapeutic and subtherapeutic concentrations of chlortetracycline.

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 micrograms 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 micrograms/g for 14 d) and growth-promoting (27.5 micrograms/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.