The individual contributions of blaB, blaGOB and blaCME on MICs of β-lactams in Elizabethkingia anophelis.

backgroundThe blaB, blaGOB and blaCME genes are thought to confer β-lactam resistance to Elizabethkingia anophelis, based on experiments conducted primarily on Escherichia coli.

objectivesTo determine the individual contributions of β-lactamase genes to increased MICs in E. anophelis and to assess their impact on the in vivo efficacy of carbapenem therapy.

methodsScarless gene deletion of one or more β-lactamase gene(s) was performed in three clinical E. anophelis isolates. MICs were determined by broth microdilution. Hydrolytic activity and expressions of β-lactamase genes were measured by an enzymatic assay and quantitative RT-PCR, respectively. In vivo efficacy was determined using Galleria mellonella and murine thigh infection models.

resultsThe presence of blaB resulted in >16-fold increases, while blaGOB caused 4-16-fold increases of carbapenem MICs. Hydrolysis of carbapenems was highest in lysates of blaB-positive strains, possibly due to the constitutionally higher expression of blaB. Imipenem was ineffective against blaB-positive isolates in vivo in terms of improvement of the survival of wax moth larvae and reduction of murine bacterial load. The deletion of blaB restored the efficacy of imipenem. The blaB gene was also responsible for a >4-fold increase of ampicillin/sulbactam and piperacillin/tazobactam MICs. The presence of blaCME, but not blaB or blaGOB, increased the MICs of ceftazidime and cefepime by 8-16- and 4-8-fold, respectively.

conclusionsThe constitutionally and highly expressed blaB gene in E. anophelis was responsible for increased MICs of carbapenems and led to their poor in vivo efficacy. blaCME increased the MICs of ceftazidime and cefepime.