Effect of chlortetracycline on the distribution of resistance genes

Abstract

The objective of this study was to compare the distribution of tetracycline resistance genes in the Gram-negative fecal flora between swine that were fed subtherapeutic chlortetracy- cline as compared to pigs that did not receive in feed antimicrobials. Within each of 2 farms, 8 fin- isher barns were temporarily matched in pairs based on pig placement dates. For each matched pair, a barn was assigned to either treatment (50g chlortetracycline/ton of feed) or control. From 48 pigs per barn, 2 Gram negative colonies from the fecal flora were selected per pig for tetracy- cline resistance gene detection. The isolates were assayed for tet (A), (B), (C), (D), (E), and (G). A total of 88.7% of isolates (805/908) harboured at least one of the genes assayed. Overall, 35.3%, 59.9%, 0.9% of isolates were identified to harbour tet (A), (B), and (C), respectively. There were no differences detected in tetracycline resistance genes distribution between treatments. Results Overall, 908 isolates were assayed. A total of 88.7% of isolates (805/908) harboured at least one of the tetracycline resistance genes assayed. Overall, 35.3%, 59.9%, 0.9% of isolates were identified to harbour tet (A), (B), and (C), respectively. Both tet (A) and tet (B) were identified in 7.5% of isolates. For those swine that received chlortetracycline, 37.5%, 60.6%, 1.7% were identified to harbour tet (A), (B), and (C), respectively. For those swine that received no antibiotics 33.1%, 59.2%, 0.2% were identified to harbour tet (A), (B), and (C) respectively. There was no significant effect of treatment on the distribution of tetracycline resistance determinants (p>0.10). Discussion These results suggest that subtherapeutic chlortetracycline, at least under short-term exposure, does not alter the distribution of tetracycline resistance genes in the fecal flora of swine. The predominant identification of tet (A), (B), and (C) in the Gram-negative fecal flora of swine is in agreement with previous reports. Although our results cannot discredit the potential