Characterisation of mcr-4.3 in a colistin-resistant Acinetobacter nosocomialis clinical isolate from Cape Town, South Africa

Abstract

Colistin resistance in Acinetobacter spp. is increasing, resulting in potentially untreatable nosocomial infections. Plasmid-mediated colistin resistance is of particular concern due to its low fitness cost and potential transferability to other bacterial strains and species. This study investigated the colistin resistance mechanism in a clinical Acinetobacter nosocomialis isolate from Cape Town, South Africa. A colistin-resistant A. nosocomialis isolate was identified from a blood culture in 2017. PCR and Illumina whole-genome sequencing (WGS) were performed to identify genes and mutations conferring resistance to colistin. Plasmid sequencing was performed on an Oxford Nanopore platform. mcr functionality was assessed by broth microdilution after cloning the mcr gene into pET-48b(+) and expressing it in SHuffle® T7 Escherichia coli and after curing the plasmid using 62.5 mg/L acridine orange. The colistin minimum inhibitory concentration (MIC) of the A. nosocomialis isolate was 16 mg/L. The mcr-4.3 gene was detected by PCR and WGS. No other previously described colistin resistance mechanism was found by WGS. The mcr-4.3 gene was identified on a 24 024-bp RepB plasmid (pCAC13a). Functionality studies showed that recombinant mcr-4.3 did not confer colistin resistance in E. coli. However, plasmid curing of pCAC13a restored colistin susceptibility in A. nosocomialis. We describe the first detection of a plasmid-mediated mcr-4.3 gene encoding colistin resistance in A. nosocomialis and the first detection of mcr-4.3 in a clinical isolate in Africa. Recombinant expression of mcr-4.3 did not confer colistin resistance in E. coli, suggesting that its functionality may be RepB plasmid-dependent or species-specific.