Continuous feeding of antimicrobial growth promoters to commercial swine during the growing/finishing phase does not modify faecal community erythromycin resistance or community structure

Aims: To investigate the effect of continuous feeding of antimicrobial growth promoters (tylosin or virginiamycin) on the swine faecal community. Methods and Results: The study consisted of two separate on‐farm feeding trials. Swine were fed rations containing tylosin (44 or 88 mg kg^?1 of feed) or virginiamycin (11 or 22 mg kg^?1 of feed) continuously over the growing/finishing phases. The temporal impact of continuous antimicrobial feeding on the faecal community was assessed and compared to nondosed control animals through anaerobic cultivation, the analysis of community 16S rRNA gene libraries and faecal volatile fatty acid content. Feeding either antimicrobial had no detectable effect on the faecal community. Conclusions: Erythromycin methylase genes encoding resistance to the macrolide–lincosamide–streptogramin B (MLS_B) antimicrobials are present at a high level within the faecal community of intensively raised swine. Continuous antimicrobial feeding over the entire growing/finishing phase had no effect on community erm‐methylase gene copy numbers or faecal community structure. Significance and Impact of the Study: Antimicrobial growth promoters are believed to function by altering gut bacterial communities. However, widespread MLS_B resistance within the faecal community of intensively raised swine likely negates any potential effects that these antimicrobials might have on altering the faecal community. These findings suggest that if AGP‐mediated alterations to gut communities are an important mechanism for growth promotion, it is unlikely that these would be associated with the colonic community.