From farm to fork: identical clones and Tn6674-like elements in linezolid-resistant Enterococcus faecalis from food-producing animals and retail meat.

Objectives: Increasing numbers of linezolid-resistant Enterococcus carrying optrA are being reported across different niches worldwide. We aimed to characterize the first optrA-carrying Enterococcus faecalis obtained from food-producing animals and retail meat samples in Tunisia.

Methods: Seven optrA-carrying E. faecalis obtained from chicken faeces (n=3, August 2017) and retail chicken meat (n=4, August 2017) in Tunisia were analysed. Antimicrobial susceptibility was determined by disc diffusion, broth microdilution and Etest against 13 antibiotics, linezolid and tedizolid, respectively (EUCAST/CLSI). optrA stability (∼600 bacterial generations), transfer (filter mating) and location (S1-PFGE/hybridization) were characterized. WGS (Illumina-HiSeq) was done for four representatives that were analysed through in silico and genomic mapping tools.

Results: Four MDR clones carrying different virulence genes were identified in chicken faeces (ST476) and retail meat (the same ST476 clone plus ST21 and ST859) samples. MICs of linezolid and tedizolid were stably maintained at 8 and 1-2 mg/L, respectively. optrA was located in the same transferable chromosomal Tn6674-like element in ST476 and ST21 clones, similar to isolates from pigs in Malaysia and humans in China. ST859 carried a non-conjugative plasmid of ∼40 kb with an impB-fexA-optrA segment, similar to plasmids from pigs and humans in China.

Conclusions: The same chromosomal and transferable Tn6674-like element was identified in different E. faecalis clones from humans and animals. The finding of retail meat contaminated with the same linezolid-resistant E. faecalis strain obtained from a food-producing animal highlights the potential role of the food chain in the worrisome dissemination of optrA that can be stably maintained without selective pressure over generations.