Effect of tylosin used as a growth promoter on the occurrence of macrolide-resistant enterococci and staphylococci in pigs.

This study was conducted to investigate the effect of tylosin used as a growth promoter on the occurrence of macrolide-resistant enterococci and staphylococci in pigs. Two identical feeding experiments with 10 pigs each were conducted to evaluate the effect of tylosin on the occurrence of erythromycin resistance among enterococci sampled from feces and Staphylococcus hyicus sampled by skin swabs. Half of the pigs were fed antibiotic-free feed and the other half feed with tylosin (30 microg/g) added. For each pig, weekly fecal samples and skin swabs were collected and examined for erythromycin-susceptible and erythromycin-resistant enterococci and S. hyicus, respectively. There was an immediate effect of tylosin on the fraction of resistant enterococci recovered, increasing by a factor 2.4; the effect on S. hyicus was more gradual, increasing at a rate of about 8% per day and totaling a five-fold increase over 20 days. Thus, a clear effect on resistance occurrence was demonstrated, not only in intestinal bacteria, but also on bacteria sampled from the skin.