Detection of Antibiotic Resistance in Swine Production

Since antibiotics have been added to animal feed for decades, food animals and their wastes constitute a reservoir of antibiotic-resistant bacteria. At the Swine Complex of McGill University, the addition of antibiotics to swine feed for subtherapeutic applications has been discontinued since January 2007. The objective of this work was to assess the prevalence and short-term evolution of antibiotic resistance among bacterial populations in swine production 2.5 years after this discontinuation. Feces from ten healthy pigs (6 males and 4 females) born at the Swine Complex of McGill from the same sow and administered feed without antibiotics were sampled during suckling, weanling, growing and finishing. The percentage of chlortetracycline-resistant anaerobic bacterial populations (TetR) was higher than that of tylosin-resistant anaerobic bacterial populations (TylR) at weanling, growing and finishing, with generally larger differences in males than in females. At the finishing stage, i.e. prior to the transportation of animals to the slaughterhouse, resistant populations varied between 3.1x10 6 and 2.5x109 CFU g-1. In all pigs, tet(L), tet(O) and erm(B) were detected by PCR at suckling and weanling, whereas only tet(O) was detected at growing and finishing. Quantification of tet(O) by real-time PCR showed that at suckling, the abundance of this gene was 18 times higher in females than in males, was similar between the two genders at weanling and growing, and reached 5.1x105 and 5.6x10 5 copies of tet(O)/etag of total DNA in the feces of males and females, respectively, at finishing. In this study, the high abundance and proportion of antibiotic-resistant populations, as well as the occurrence of resistance genes within these populations despite the discontinuation of antibiotic addition to feeds imply either that more time would be required for antibiotic resistance to decrease to lower levels, and/or that factors such as the presence of metals in feed impose a selective pressure that maintains antibiotic resistance genes among these bacterial populations.