Prolonged outbreak of multidrug-resistant Shigella sonnei harbouring blaCTX-M-27 in Victoria, Australia.

Objectives: In Australia, cases of shigellosis usually occur in returned travellers from shigellosis-endemic regions, or in men who have sex with men. Resistance to multiple antibiotics has significantly increased in Shigella sonnei and represents a significant public health concern. Here we investigate an outbreak of multidrug-resistant S. sonnei in Victoria, Australia.Methods: We undertook whole genome sequencing of 54 extended-spectrum beta-lactamase (ESBL) producing S. sonnei received at the Microbiological Diagnostic Unit Public Health Laboratory between January 2019 and March 2020. The population structure and antimicrobial resistance profiles were identified by genomic analyses, with 73 previously characterised Australian S. sonnei to provide context. Epidemiological data including age and sex of the shigellosis cases were also collected.Results: There was a significant increase in cases of ESBL S. sonnei from July 2019. Most of the ESBL S. sonnei (65%) fell within a single cluster, that was predominantly comprised of male cases, and were characterised by the presence of blaCTX-M-27 gene conferring resistance to extended-spectrum cephalosporins. These isolates were also multidrug-resistant, including resistance to azithromycin and co-trimoxazole and reduced susceptibility to ciprofloxacin.Conclusions: Our data has uncovered a prolonged clonal outbreak of ESBL S. sonnei that was likely first introduced by returned travellers and has subsequently been circulating locally in Australia. The emergence of a local outbreak of ESBL S. sonnei, with a multidrug-resistant profile, including reduced susceptibility to ciprofloxacin, represents a significant public health threat.