Plasmid-mediated bla_GES-24 in Acinetobacter nosocomialis: Genetic context and resistance profile

A clinical isolate of Acinetobacter spp. MS5394 carrying a novel GES-type β-lactamase gene, bla_GES-24, was first identified in 2013. This study aimed to characterize the genetic and phenotypic features of GES-24.

The complete genome of A. nosocomialis MS5394 was sequenced using Illumina MiSeq and Oxford Nanopore technologies. Plasmid pAC-GES-24 was analysed for resistance gene content and sequence. Site-directed mutagenesis was used to examine the functional role of amino acid substitutions at positions 62, 104, and 170 in GES-24. Antimicrobial susceptibility testing was performed in E. coli expressing different GES variants.

The bla_GES-24, is on a ∼153-kb plasmid pAC-GES-24. The complete nucleotide sequence of the pAC-GES-24 indicated that the backbone sequence was possibly a hybrid of two independent plasmids, pNDM-AP_882 and pOXA-588-AP_882, carrying bla_NDM and bla_OXA, respectively, from Acinetobacter pitti AP_882 isolated in Malaysia. A comparison of the amino acid sequence of GES-24 with that of highly homologous GES identified three amino acid differences at ambler positions 62, 104, and 170. Phenotypic investigation of E. coli producing GES-24 in comparison with E. coli producing GES variants at Ambler positions 62, 104, and 170 under isogenic conditions confirmed that GES-24 is a β-lactamase that possesses carbapenemase activity but is relatively weak in hydrolysing CTX, CAZ, AZT, and CFPM when compared to the other GES-type tested.

GES-24 represents a GES-type β-lactamase with carbapenemase activity but relatively weak hydrolytic activity against certain β-lactams. The genetic analysis suggests that the plasmid carrying bla_GES-24 may have evolved from a hybrid plasmid.