Technical note: Occurrence in fecal microbiota of genes conferring resistance to both macrolide-lincosamide-streptogramin B and tetracyclines concomitant with feeding of beef cattle with tylosin

¹ Department of Animal Sciences, The Ohio State University, Columbus, OH 43210
² Department of Animal Sciences, The Ohio State University, Columbus, OH 43210; CSIRO Livestock Industries, St Lucia, Australia

^* To whom correspondence should be addressed. E-mail: yu.226{at}osu.edu.

	Abstract

Development of antimicrobial resistance in food animals receiving antimicrobials has been well documented among bacterial isolates, especially pathogens, but information on development of antimicrobial resistance at microbial community level during long-term feeding of antimicrobials is lacking. The objective of this study was to examine the association between inclusion of tylosin in feed and occurrence of resistance to macrolide-lincosamide-streptogramin B (MLS_B) in the entire fecal microbial communities of beef cattle over a feeding study of 168 d. A completely randomized design included 6 pens housed together in 1 barn, with each pen housing 10 to 11 steers. The Control and the Tylosin groups each had 3 pens, with the former receiving no antimicrobial while the latter received both tylosin and monensin (11 mg/kg and 29.9 mg/kg of feed, respectively, DM) in feed. The abundance of genes conferring resistance to MLS_B (erm genes) and tetracyclines (tet genes) were quantified using class-specific real-time PCR assays. The abundances of erm and tet genes were analyzed with pens as experimental units using the MIXED procedure of SAS. Correlations between abundance of different resistance genes were calculated using the CORR procedure of SAS. We identified 4 classes (B, F, T, and X) of erm genes in fresh fecal samples collected at wk 2, 17, and 21 of feeding. From wk 2 to 17, the abundance of erm(T) and erm(X) increased (P < 0.05), while that of erm(B) and erm(F) did not. The abundance of the erm genes did not further change from wk 17 to 21. tet(A/C), tet(G), and tet genes encoding ribosomal protection proteins (including classes M, O, P, Q, S, T, and W) appeared to be co-selected by tylosin feeding. Such co-selection of multiresistance at community level by 1 antimicrobial drug used in animals has the important implication that future studies should examine resistance to not only the antimicrobials used in animals, but also other antimicrobials, especially those used in human medicine, to fully assess the potential risk associated with antimicrobial use in animals. Both the erm and tet genes appeared to be disseminated among the microbial populations in all steers housed together.

Key Words: antimicrobial resistance, erm, real-time PCR, tet, tylosin