Deep genomic characterisation of an extensively drug-resistant <i>Shigella sonnei</i> carrying bla <i> _CTX-M-15 </i> in a sexually transmitted infection in an MSM patient in Spain

Objectives We aimed to characterise the genomic features, resistance determinants, plasmid content and phylogenetic relatedness of an extensively drug-resistant (XDR) Shigella sonnei isolate carrying bla CTX-M-15 identified in northern Spain. Methods A 31-year-old man who has sex with men (MSM) living with HIV on antiretroviral therapy with a prior history of treated syphilis presented with 1 week of bloody diarrhoea and abdominal pain. He reported multiple recent sexual partners and use of mobile dating applications and had no symptoms suggestive of urethritis or proctitis. Whole-genome sequencing was conducted and phylogenetic relatedness to international bla CTX-M-15 S. sonnei strains were assessed using single nucleotide polymorphism (SNP) based analysis. Results As symptoms persisted, the patient re-attended the emergency department and was discharged after Infectious Diseases consultation while awaiting susceptibility results. Whole-genome sequencing assigned it to ST152 and exhibited resistance to third-generation cephalosporins, aminoglycosides, ciprofloxacin, trimethoprim–sulfamethoxazole and azithromycin while retaining susceptibility to amoxicillin–clavulanic acid, nitrofurantoin and fosfomycin. Oral fosfomycin 500 mg was subsequently prescribed in primary care. A follow-up stool culture 1 month later was negative. Phylogenetic analysis demonstrated that this isolate was genetically distinct from clusters reported in England and Los Angeles and only moderately related to Spanish strains from Bilbao, suggesting an independent acquisition event. Conclusions We describe the second report in Spain of an XDR S. sonnei harbouring bla CTX-M-15 sexually transmitted in an MSM. The coexistence of multiple resistance genes on potentially conjugative plasmids highlights the capacity of this pathogen as vehicles for resistance gene dissemination, which underscores the public health threat posed by sexually transmitted XDR S. sonnei .