First report of an Enterobacter cloacae ST837 resistant to cefiderocol co-harbouring blaIMP-19 and mcr-4.3 resistance genes in Italy

Objective: To describe the molecular characterisation of an Enterobacter cloacae strain resistant to cefiderocol and co-harbouring blaIMP-19 and mcr-4.3 resistance genes Methods: The strain was isolated from the rectal swab of a 77-year-old woman during screening for the detection of patients colonised by carbapenemase-producing Enterobacterales (CPE) in Bologna (Italy). The strain was identified at species level by MALDI-TOF mass spectrometry and the presence of active-on-imipenem (IMP) carbapenemase was confirmed both by a phenotypic immunochromatographic method, and by a rapid commercial nucleic acid amplification test. The strain underwent antimicrobial susceptibility testing and its genome was fully characterised to assess (i) the multilocus sequence type, (ii) the presence of antimicrobial resistance (AMR) genes, and (iii) the presence of virulence genes coupled with mobile genetic elements. Results: Multilocus sequence typing identified the strain as ST837. A plasmid of 60 691 bp, very similar to the MW574937 plasmid of Escherichia coli, was identified and carried the following AMR genes: aac(6′)-II, blaBEL-1, blaIMP-19, msr(E), mph(E), and sul1. This plasmid harboured several tra genes (traB, traC, traD, traG, traI, traJ, traKtraL, and traM) of the transfer operon of the E. coli conjugative F plasmid. Other AMR genes were located in the chromosome (blaCMH-3 and fosA) or in other plasmids (blaSHV-12 and mcr-4.3). The strain showed resistance to many β-lactams in vitro, including cefiderocol, whereas, despite the presence of the mcr-4.3 gene, it was susceptible to colistin. Conclusions: Further studies are needed to better evaluate the origin of this strain and monitor the presence of Gram-negative bacilli with a similar molecular structure in our area.