Abstract
Tylosin phosphate is a macrolide commonly administered to cattle in North America for the control of liver abscesses. This study investigated the effect of in-feed administration of tylosin phosphate to cattle at subtherapeutic levels and its subsequent withdrawal on macrolide resistance using enterococci as an indicator bacterium. Fecal samples were collected from steers that received no antibiotics and steers administered tylosin phosphate (11 ppm) in-feed for 197 days and withdrawn 28 days before slaughter. Enterococcus species isolated from fecal samples were identified through sequencing the groES-EL intergenic spacer region and subject to antimicrobial susceptibility testing, identification of resistance determinants and pulsed-field gel electrophoresis profiling. Tylosin increased (P < 0.05) the proportion of eryR and tylR enterococci within the population. Just prior to its removal, the proportion of eryR and tylR resistant enterococci began decreasing and continued to decrease after tylosin was withdrawn from the diet until there was no difference (P > 0.05) between treatments on d 225. This suggests that antibiotic withdrawal prior to slaughter contributes to a reduction in the proportion of macrolide resistant enterococci entering the food chain. Among the 504 enterococci isolates characterized, Enterococcus hirae was found to predominate (n = 431), followed by Enterococcus villorum (n = 32), Enterococcus faecium (n = 21), Enterococcus durans (n = 7), Enterococcus casseliflavus (n = 4), Enterococcus mundtii (n = 4), Enterococcus gallinarum (n = 3), Enterococcus faecalis (n = 1), and Enterococcus thailandicus (n = 1). The diversity of enterococci was greater in steers at arrival than at exit from the feedlot. Erythromycin resistant isolates harbored the erm(B) and/or msrC gene. Similar PFGE profiles of eryR E. hirae pre- and post-antibiotic treatment suggest that increased abundance of eryR enterococci after administration of tylosin phosphate reflects selection for strains that were already present within the gastrointestinal tract of cattle at arrival.
Highlights
Subtherapeutic administration of antibiotics in livestock feed has come under increasing scrutiny due to concerns that such a practice increases the emergence of antibiotic resistant bacteria (Aarestrup, 1999)
If the inclusion of tylosin in feed leads to tylosin resistant enterococci in cattle it may select for enterococci that are resistant to other macrolides such as erythromycin, an antibiotic important for the treatment of bacterial infections in humans (Roberts, 2008; Desmolaize et al, 2011)
Few studies have investigated the role that administration of tylosin in the feed of beef cattle has on the development of macrolide resistance in enterococci
Summary
Subtherapeutic administration of antibiotics in livestock feed has come under increasing scrutiny due to concerns that such a practice increases the emergence of antibiotic resistant bacteria (Aarestrup, 1999). This concern is relevant for bacteria that reside in livestock and are associated with clinical infections in humans. If the inclusion of tylosin in feed leads to tylosin resistant enterococci in cattle it may select for enterococci that are resistant to other macrolides such as erythromycin, an antibiotic important for the treatment of bacterial infections in humans (Roberts, 2008; Desmolaize et al, 2011)
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