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 anti-microbial linezolid have emerged among food -produc-ing 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 oxa-zolidinone resistance genes. Methods: Fifty-nine sam-ples of different types of RMBDs from 10 suppliers (three based in Germany, seven in Switzerland) were screened for florfenicol-resistant Gram-positive bacte-ria using a selective culture medium. Isolates were phe-notypically and genotypically characterised. Results: A total of 27 Enterococcus faecalis, Enterococcus fae-cium, 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 encod-ing 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.