Fosfomycin (FOS) is an effective antibiotic against multidrug-resistant Enterobacterales, but its effectiveness is reducing. Little is known on the current prevalence of FosA enzymes in low-risk pathogens, such as Citrobacter freundii. The aim of the study was the molecular characterization of a carbapenemase- and FosA-producing C. freundii collected in Italy. AK867, collected in 2023, showed an XDR profile, retaining susceptibility only to colistin. AK867 showed a FOS MIC >128 mg/L by ADM. Based on WGS, AK867 belonged to ST116 and owned a wide resistome, including fosA3, blaKPC-2, and blaVIM-1. fosA3 was carried by a conjugative pKPC-CAV1312 plasmid of 320,480 bp, on a novel composite transposon (12,907 bp). FosA3 transposon shared similarities with other fosA3-harboring pKPC-CAV1312 plasmids among Citrobacter spp. We report the first case of FosA3 production in clinical carbapenemase-producing C. freundii ST116. The incidence of FosA3 enzymes is increasing among Enterobacterales, affecting even low-virulence pathogens, as C. freundii.

1 R C C S Policlinico S Matteo Pavia Italy

Biomedical Center Faculty of Medicine Charles University Pilsen Czechia

Clinical Microbiology Azienda Unità Sanitaria Locale Modena Modena Italy

Department of Microbiology Faculty of Medicine University Hospital in Pilsen Charles University Pilsen Czechia

Infectious Diseases Clinic Azienda Ospedaliera Universitaria Policlinico di Modena Modena Italy

Microbiology and Clinical Microbiology Unit Scienze Clinico Chirurgiche Diagnostiche Pediatriche Department University of Pavia Pavia Italy

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