The effect of chlortetracycline treatment and its subsequent withdrawal on multi-resistant Salmonella enterica serovar Typhimurium DT104 and commensal Escherichia coli in the pig.

Aims: To investigate the effect of a therapeutic and sub-therapeutic chlortetracycline treatment on tetracycline-resistant Salmonella enterica serovar Typhimurium DT104 and on the commensal Escherichia coli in pig.
Methods and Results: Salmonella Typhimurium DT104 was orally administered in all pigs prior to antibiotic treatment, and monitored with the native E. coli. Higher numbers of S. Typhimurium DT104 were shed from treated pigs than untreated pigs. This lasted up to 6 weeks post-treatment in the high-dose group. In this group, there was a 30% increase in E. coli with a chlortetracycline minimal inhibitory concentration (MIC) > 16 mg l-1 and a 10% increase in E. coli with an MIC > 50 mg l-1 during and 2 weeks post-treatment. This effect was less-pronounced in the low-dose group. PCR identified the predominant tetracycline resistance genes in the E. coli as tetA, tetB and tetC. The concentration of chlortetracycline in the pig faeces was measured by HPLC and levels reached 80 microg g-1 faeces during treatment.
Conclusion: Chlortetracycline treatment increases the proportion of resistant enteric bacteria beyond the current withdrawal time.
Significance and Impact of the Study: Treated pigs are more likely to enter abattoirs with higher levels of resistant bacteria than untreated pigs promoting the risk of these moving up the food chain and infecting man.