Antibiotic resistance in Escherichia coli and Enterococcus spp. isolates from commercial broiler chickens receiving growth-promoting doses of bacitracin or virginiamycin.

Antibacterial agents such as zinc bacitracin (ZB) and virginiamycin (VG) are used as growth promoting agents (GP) in broiler chicken production. The objective of this study was to evaluate the effect of the use of ZB and VG on the emergence of antibacterial resistance in a commercial broiler chicken farm. Three trials were each conducted using three different diets: one without antibacterial agents, one containing VG and one with ZB. Escherichia coli and Enterococcus spp strains were isolated and tested for their susceptibility to various antibacterial agents. The occurence of the resistance genes vatD, ermB and bcrR in Enterococcus spp. isolates was determined by PCR. Comparative quantification of vatd and bcrR genes in total DNA extracts from litter was done by SYBR Green Real-Time PCR (QPCR). E. coli and Enterococcus spp. isolates from diet groups had altered levels of resistance to various antibacterial agents over time. These GPs did not select for specific antibacterial agent resistance (AAR) in Enterococcus spp. The use of GPs seemed to lower the percentage of E. coli, isolates resistant to some antibacterial agents. The presence of bcrR gene could not explain all resistant phenotypes to ZB. Other genes than vatD and ermB seemed to be involved in the resistance to VG. Use of GPs was not associated with bcrR gene in DNA extracts from litter but use of VG was associated with vatD presence.