High occurrence of Enterococcus faecalis, Enterococcus faecium, and Vagococcus lutrae harbouring oxazolidinone resistance genes in raw meat-based diets for companion animals – a public health issue, Switzerland, September 2018 to May 2020

INTRODUCTION: Enterococci harbouring genes encoding resistance to florfenicol and the oxazolidinone antimicrobial linezolid have emerged among food-producing animals and meat thereof, but few studies have analysed their occurrence in raw meat-based diets (RMBDs) for pets. AIM: We aimed to examine how far RMBDs may represent a source of bacteria with oxazolidinone resistance genes. METHODS: Fifty-nine samples of different types of RMBDs from 10 suppliers (three based in Germany, seven in Switzerland) were screened for florfenicol-resistant Gram-positive bacteria using a selective culture medium. Isolates were phenotypically and genotypically characterised. RESULTS: A total of 27 Enterococcus faecalis, Enterococcus faecium, and Vagococcus lutrae isolates were obtained from 24 of the 59 samples. The optrA, poxtA, and cfr genes were identified in 24/27, 6/27 and 5/27 isolates, respectively. Chloramphenicol and linezolid minimum inhibitory concentrations (MICs) ranged from 24.0 mg/L–256.0 mg/L, and 1.5 mg/L–8.0 mg/L, respectively. According to the Clinical and Laboratory Standards Institute (CLSI) breakpoints, 26 of 27 isolates were resistant to chloramphenicol (MICs ≥ 32 mg/L), and two were resistant to linezolid (MICs ≥ 8 mg/L). Multilocus sequence typing analysis of the 17 E. faecalis isolates identified 10 different sequence types (ST)s, with ST593 (n = 4 isolates) and ST207 (n = 2 isolates) occurring more than once, and two novel STs (n = 2 isolates). E. faecium isolates belonged to four different STs (168, 264, 822, and 1846). CONCLUSION: The high occurrence in our sample of Gram-positive bacteria harbouring genes encoding resistance to the critical antimicrobial linezolid is of concern since such bacteria may spread from companion animals to humans upon close contact between pets and their owners.