Resistance to ceftazidime-avibactam is due to transposition of KPC in a porin-deficient strain of Klebsiella pneumoniae with increased efflux activity. [PublishAheadOfPrint]

Ceftazidime-avibactam is an antibiotic with activity against serine beta-lactamases, including KPC. Recently, reports have emerged of KPC-producing isolates resistant to this antibiotic, including a report of a wild-type KPC-3 producing sequence type 258 Klebsiella pneumoniae that was resistant to ceftazidime-avibactam. We describe a detailed analysis of this isolate, in the context of two other closely related KPC-3 producing isolates, recovered from the same patient. Both isolates encoded a non-functional OmpK35, whereas we demonstrate a novel T333N mutation in OmpK36, present in the ceftazidime-avibactam resistant isolate, reduced activity of this porin and impacted ceftazidime-avibactam susceptibility. Additionally, we demonstrate that the increased expression of bla_KPC-3 and bla_SHV-12 observed in the ceftazidime-avibactam resistant isolate was due to transposition of the Tn4401 transposon harboring bla_KPC-3 into a second plasmid, pIncX3, which also harbored bla_SHV-12, ultimately resulting in a higher copy number of bla_KPC-3 in the resistant isolate. pIncX3 plasmid from the ceftazidime-avibactam resistant isolate, conjugated into a OmpK35/36 deficient K. pneumoniae background that harbored mutation to the ramR regulator of the acrAB efflux operon re-created the ceftazidime-avibactam resistant MIC of 32 μg/mL, confirming this constellation of mutations is responsible for the resistance phenotype.

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