Date Full Report Received


Date Abstract Report Received



Primary Investigator:

 This study examined the impact of subtherapeutic and therapautic levels of chlortetracycline in feed on the selection and dissemination of mobile genetic elements called IncA/C plasmids among E. coli in the pig gut. These plasmids are important to human and animal health because they enable bacteria to become resistant to multiple antibiotics, and they can be spread between bacteria through conjugation. Using oral inoculation of pigs with bacteria containing IncA/C plasmids, we showed that subtherapeutic use of chlortetracycline had no impact on the selection of these plasmids over the course of six weeks. In contrast, therapeutic use of the same drug significantly increased the proportion of plasmid-containing E. coli in the pig gut. Also, the mobility of IncA/C plasmids to different bacterial hosts was not evident from our work, suggesting that their movement is not as extensive as previously thought. This work indicates that short-term use of chlortetracycline as a growth promoter in feed does not necessarily select for multidrug resistance encoded by IncA/C-type plasmids in E. coli.