Characteristics of linezolid-resistant Enterococcus faecalis isolates from broiler breeder farms

Abstract

Linezolid is an oxazolidinone class antibiotic used for treatment infections caused by various multidrug-resistant gram-positive pathogens including enterococci. However, recently, linezolid-resistant isolates in animals are considered as a human health hazard. In a broiler operation system, antimicrobial resistance can be transferred to the environment and commercial broiler via the fecal-oral route. Therefore, this study was conducted to investigate the prevalence and characteristics of linezolid-resistant Enterococcus faecalis (E. faecalis) from broiler parent stock in a broiler operation system. Among 297 E. faecalis isolates from 85 flocks in 8 broiler breeder farms, the prevalence of chloramphenicol- and linezolid-resistant isolates was 0 to 12.1% and 0 to 8.0%, respectively; however, there were no significant differences between farms. Therefore, a total of 14 (4.7%) chloramphenicol- and/or linezolid-resistant E. faecalis showed resistance to 7 or more antimicrobial classes. The drug-resistance gene optrA, which can confer resistance to linezolid, tedizolid, and phenicols, was found in 8 (2.69%) isolates, and 7 (2.36%) of the 8 optrA-positive isolates co-carried the phenicol exporter gene fexA. However, E. faecalis isolates from 3 of 8 broiler breeder farms only carried the optrA and/or fexA genes. As linezolid is one of the last antimicrobial treatments of choice for multidrug-resistant gram-positive pathogens including E. faecalis, the presence of antibiotic-resistant E. faecalis in broiler breeder farms should be monitored to prevent the introduction of linezolid-resistant strains to the food chain.

Keywords: Enterococcus faecalis; antimicrobial resistance; broiler breeder; linezolid-resistant; optrA.

Figure 1

Dendrogram of SmaI-PFGE patterns of 8 optrA-positive E. faecalis isolates. E. faecalis isolates showing similarities of <85% were considered to be unrelated. Underline indicated that was found in the transconjugant strains. ^aResistance gene cfr was tested but not detected. ^bLZD, linezolid; CHL, chloramphenicol; FFN, florfenicol; CIP, ciprofloxacin; DAP, daptomycin; ERY, erythromycin; GEN, gentamicin; KAN, kanamycin; SYN, quinupristin/dalfopristin; SAL, salinomycin; STR, streptomycin; TET, tetracycline; TYLT, tylosin tartrate. Abbreviations: MIC, minimum inhibitory concentrations.