Frequency of antibiotic resistance in a swine facility 2.5 years after a ban on antibiotics.

The addition of antibiotics to livestock feed has contributed to the selection of antibiotic-resistant bacteria in concentrated animal feeding operations and agricultural ecosystems. The objective of this study was to assess the occurrence of resistance to chlortetracycline and tylosin among bacterial populations at the Swine Complex of McGill University (Province of Quebec, Canada) in the absence of antibiotic administration to pigs for 2.5 years prior to the beginning of this study. Feces from ten pigs born from the same sow and provided feed without antibiotic were sampled during suckling (n = 6 for enumerations, n = 10 for PCR), weanling (n = 10 both for PCR and enumerations), growing (n = 10 both for PCR and enumerations), and finishing (n = 10 both for PCR and enumerations). The percentage of chlortetracycline-resistant anaerobic bacterial populations (Tet(R)) was higher than that of tylosin-resistant anaerobic bacterial populations (Tyl(R)) at weanling, growing, and finishing. Prior to the transportation of animals to the slaughterhouse, resistant populations varied between 6.5 and 9.4 Log colony-forming units g humid feces(-1). In all pigs, tet(L), tet(O), and erm(B) were detected at suckling and weanling, whereas only tet(O) was detected at growing and finishing. The abundance of tet(O) was similar between males and females at weanling and growing and reached 5.1 × 10(5) and 5.6 × 10(5) copies of tet(O)/ng of total DNA in males and females, respectively, at finishing. Results showed high abundances and proportions of Tet(R) and Tyl(R) anaerobic bacterial populations, as well as the occurrence of tet and erm resistance genes within these populations despite the absence of antibiotic administration to pigs at this swine production facility since January 2007, i.e., 2.5 years prior to the beginning of this study. This work showed that the occurrence of bacterial resistance to chlortetracycline and tylosin is high at the Swine Complex of McGill University.