High-level and novel mechanisms of carbapenem resistance in Gram-negative bacteria from tertiary hospitals in Nigeria

To determine the occurrence and molecular basis of carbapenem resistance in Gram-negative bacteria from tertiary hospitals in Nigeria, 182 non-duplicate Gram-negative bacterial isolates were investigated for antimicrobial susceptibility, presence of carbapenemases (tested phenotypically and genotypically), random amplified polymorphic DNA (RAPD) typing, plasmid sizing and replicon typing. Minimum inhibitory concentrations of carbapenems showed a high degree of resistance, with 67 isolates (36.8%) being resistant to all carbapenems, of which 40 (59.7%) produced enzymes able to hydrolyse imipenem. PCR and sequencing identified only 10 isolates (5.5%) carrying known carbapenemase genes, including bla_NDM, bla_VIM and bla _GES. The majority of phenotypically carbapenem-resistant and carbapenemase-producing isolates did not carry a known carbapenemase gene. Transconjugant or transformant plasmid sizes were estimated to be 115 kb for bla_NDM- and 93 kb for bla_VIM-carrying plasmids. These plasmids were untypeable for replicon/incompatibility and transferred various other genes including plasmid-mediated quinolone resistance (PMQR) genes and bla_CTX-M-15. Typing showed that the isolates in this study were not clonally related. There is a high level of carbapenem resistance in Nigeria. As well as the globally relevant carbapenemases (bla_NDM, bla _VIM and bla_GES), there are other unknown gene(s) or variant(s) in circulation able to hydrolyse carbapenems and confer high-level resistance.