Development of macrolide resistant Campylobacter in broilers administered tylosin and the genetic characterization of the associated mutations
Ladely, Scott R.
MetadataShow full item record
Antimicrobial use in food animal production has become a source of controversy in recent years. However, limited data are available regarding such use and the development of antimicrobial resistance in bacterial species. The effect of tylosin administration on erythromycin susceptibility of Campylobacter isolated from broiler ceca was evaluated. In each of three replicate studies, broilers exposed to macrolide-susceptible strains of C. jejuni or C. coli were administered tylosin at subterapeutic or therapeutic levels. Broilers were sacrificed weekly, and total and resistant Campylobacter were enumerated from individual ceca. Erythromycin resistance was observed at a higher frequency (P<0.01) among C. coli (70.8%) compared to C. jejuni isolates (36.8%). Across Campylobacter species erythromycin resistance was observed at a higher frequency (P<0.001) when tylosin was administered at subtherapeutic (62.7%), compared to therapeutic levels (11.4%). To further characterize development of macrolide resistance in Campylobacter, the effect of operon specific 23S rRNA gene mutations on minimum inhibitory concentrations (MICs) of macrolides was examined in a collection of Campylobacter. Sequencing data for domain V of the 23S rRNA gene showed that macrolide-resistant C. coli isolates had A2059G transitions (E. coli numbering). This mutation was also observed among C. jejuni isolates. One C. coli and one C. jejuni isolate had A2059G transitions in only two of the three copies of the target gene. Isolates with only two copies of the target gene mutated had MICs one-half the concentration for erythromycin, azithromycin, tylosin and clindamycin compared to isolates with all three copies of the target gene mutated. A separate point mutation, an A2058C transversion, present in all three copies of the 23S rRNA gene was also observed among macrolide-resistant C. jejuni isolates. Campylobacter jejuni isolates with the A2058C transversion had higher erythromycin MICs (>256 ¼g/ml) compared to C. jejuni isolates with A2059G transitions (64-128 ¼g/ml). These data demonstrate that that level of tylosin administration (subterapeutic or therapeutic) and Campylobacter species influence development of macrolide resistance. Furthermore, the level of macrolide resistance resulting from point mutations in Campylobacter 23S rRNA genes may vary depending on nucleotide substitution and the number of mutated copies of the 23S rRNA gene.