What Contributes to the MIC? Beyond β-Lactamase Gene Detection in Klebsiella pneumoniae

Background K. pneumoniae is capable of resistance to β-lactam antibiotics through expression of β-lactamases (both chromosomal and plasmid-encoded) and downregulation of outer membrane porins. However, the extent to which these mechanisms interplay in a resistant phenotype is not well understood. The purpose of this study was to determine the extent to which β-lactamases and outer membrane porins affected β-lactam resistance. Methods MICs to β-lactams and inhibitor combinations were determined by agar dilution or E-test. Outer membrane porin production was evaluated by western blot of outer membrane fractions. β-lactamase carriage was determined by whole genome sequencing and expression evaluated by RT-qPCR. Results Plasmid-encoded β­-lactamases were important for cefotaxime and ceftazidime resistance. Elevated expression of chromosomal SHV was important for ceftolozane/tazobactam resistance. Loss of outer membrane porins was predictive of meropenem resistance. ESβLs and pAmpCs in addition to porin loss were sufficient to confer resistance to the third generation cephalosporins, pipercillin/tazobactam, ceftolozane/tazobactam, and meropenem. pAmpCs (CMY-2 and DHA) alone conferred resistance to pipercillin/tazobactam. Discussion Detection of a resistance gene by whole genome sequencing was not sufficient to predict resistance to all antibiotics tested. some β-lactam resistance was dependent on the expression of both plasmid-encoded and chromosomal β-lactamases and loss of porins.