Emerging strains belonging to PCR ribotype 955 in Serbia are distinct from metronidazole-resistant RT955 outbreak isolates from the UK.

End 2023, the UK Health Security Agency sent an alert about a new hypervirulent PCR ribotype, ribotype 955 (RT955), causing slowly progressing infection clusters in hospitals in the Midlands. Between March 2018 and February 2022, surveillance of infections (CDI) was performed in southern Serbia with centres providing medical services for approximately 750,000 inhabitants. Using the ECDC recommended protocol, clinical, epidemiological and microbiological data were collected. RT955 was identified in 27 (7%) of 383 surveyed patients with CDI. Of 27 patients, 16 (59%) was older than 60 years and 19 (70%) were male. CDI was always associated with previous antibiotic therapy and had a hospital onset in 23 (85%) patients. The clinical presentation was milder than reported in UK. All sequenced strains belonged to multilocus sequence type (ST) 1 and were highly similar, with 0-1 alleles differences in a core genome multilocus sequence typing analysis. The strains differed clearly from the UK RT955 outbreak strain by whole genome sequencing and phenotypic susceptibility to metronidazole, lincosamides and rifampicin. Interestingly, a high-level erythromycin resistance was observed associated with the presence of the gene. Both the UK and Serbian RT955 strains contained _p.T82I associated with resistance to fluoroquinolone antimicrobials and carried the PnimBG promoter mutation, suggestive for haem-dependent metronidazole resistance. We conclude that RT955 is present in southern Serbia since 2018. The Serbian RT955 strains differed clearly from a representative UK cluster strain.