Characterizing the Transfer of Bacterial Antibiotic Resistance Genes Across Generations of Swine

Sows and pigs were used to characterize the origin, transfer and persistence of bacterial resistance in swine. Effects of sow’s previous exposure to antibiotics and subsequent use of antibiotics in their pigs on antibiotic resistance of Salmonella enterica Typhimurium, Enterococcus faecalis, and E. coli were determined. Eight pregnant sows were divided into two groups, with four sows receiving oxytetracycline and four sows receiving no antibiotics. Fecal samples were obtained from sows prior to antibiotic exposure, and at 1- week intervals until pigs were weaned. Weaned pigs were challenged with Salmonella Typhimurium containing a nalidixic acid. Pigs from each sow treatment group were divided equally between a subtherapeutic antibiotic treatment regimen or exclusion of antibiotics. Pigs on the antibiotic treatment received apramycin at 150 g/ton of feed, beginning 7 days postweaning and lasting for 14 days, followed by oxytetracycline at 50 g/ton throughout the grow/finish period. Fecal samples were obtained from the pigs while on the sows and at 2, 7, 14, 30, 60, 114 and 115 days postweaning. The Salmonella challenge organism, E. coli and E. faecalis were recovered and tested against both apramycin and oxytetracycline using a minimum inhibitory concentration (MIC) analysis. Data were analyzed using the mixed models procedure of SAS. Polymerase Chain Reaction and transformation techniques were used to characterize genetic resistance elements and determine if the location of such gene sequences. Random apramycinresistant E. coli isolates (n = 110) were chosen from antibiotic treated sows and pigs, non-antibiotic treated sows and pigs and environmental manure to test through PCR, plasmid profiling, and macrorestriction analysis. Treatments affected antibiotic resistance to the greatest extent in E. coli, compared to Salmonella Typhimurium and Enterococcus faecalis. The greatest resistance to apramycin occurred in E. coli isolates from nursing pigs on sows that had earlier exposure to tetracyclines, and from pigs treated with apramycin during the postweaning period. Resistance to oxytetracycline was consistently high throughout the study in isolates from all pigs and sows, including those with no previous exposure to that drug. Genes responsible for apramycin resistance were found in approximately 90% of resistant isolates and their location was determined to be on bacterial plasmids. It was also determined that several different types of E. coli contained the aac(3)-IV gene responsible for apramycin resistance. These results indicate that apramycin and tetracycline resistance in E. coli was affected by previous use of tetracycline in sows (P ≥ 05). Additionally, subsequent use of antibiotics in pigs also affected (P ≤ 05) resistance levels in E. coli, whereas Salmonella Typhimurium and Enterococcus faecalis were not affected by antibiotic use in sows or pigs.

Key Words: antibiotic resistance, swine, E. coli