Molecular characterization of florfenicol and oxazolidinone resistance in isolates from animals in China.

Florfenicol is widely used for the treatment of bacterial infections in domestic animals. The aim of this study was to analyze the molecular mechanisms of florfenicol and oxazolidinone resistance in isolates from anal feces of domestic animals. The minimum inhibitory concentration (MIC) levels were determined by the agar dilution method. Polymerase chain reaction (PCR) was performed to analyze the distribution of the resistance genes. Whole-genome sequencing and comparative plasmid analysis was conducted to analyze the resistance gene environment. A total of 351 non-duplicated enteric strains were obtained. Among these isolates, 22 isolates, including 19 and 3 , were further studied. 31 florfenicol resistance genes (13 , 3 , 12 and 3 genes) were identified in 15 of the 19 isolates, and no florfenicol or oxazolidinone resistance genes were identified in 3 isolates. Whole-genome sequencing of P47, which had all four florfenicol and oxazolidinone resistance genes and high MIC levels for both florfenicol (256 mg/L) and linezolid (8 mg/L), revealed that it contained a chromosome and 3 plasmids (pP47-27, pP47-61, and pP47-180). The four florfenicol and oxazolidinone resistance genes were all related to the insertion sequences IS and located on two smaller plasmids. The genes and encoded in pP47-27, while and encoded in the conjugative plasmid pP47-61. Comparative analysis of homologous plasmids revealed that the sequences with high identities were plasmid sequences from various species except for the Tn sequence from a chromosome (MH746818.1). The current study revealed that florfenicol and oxazolidinone resistance genes (, , and ) were widely distributed in isolates from animal in China. The mobile genetic elements, including the insertion sequences and conjugative plasmid, played an important role in the horizontal transfer of florfenicol and oxazolidinone resistance.