Outer membrane protein alterations and blaTEM-1 variants: their role in beta-lactam resistance in Klebsiella pneumoniae.

OBJECTIVES: The aim of the study was to characterize the genetic basis of resistance to selected beta-lactam antibiotics in two clinical isolates of Klebsiella pneumoniae. METHODS AND RESULTS: K. pneumoniae strains were isolated from two hospitalized patients. One of the strains was resistant to amoxicillin, co-amoxiclav, cefuroxime, piperacillin and cefoxitin but susceptible to all the other cephalosporins tested. The second strain displayed a similar phenotype except that it was resistant to piperacillin/tazobactam and susceptible to cefoxitin. PCR assays and DNA sequencing showed that the cefoxitin-susceptible strain contained a novel blaTEM-1 variant downstream of the strong Pa/Pb promoter. SDS-PAGE analysis of the outer membrane proteins (OMPs) did not identify OmpK35 and suggested reduced expression of OmpK36 in this strain. Following passage in non-selective media, expression of OmpK36 was restored with a concomitant increase in cefuroxime susceptibility. A similar experimental approach identified blaTEM-1C in the cefoxitin-resistant K. pneumoniae strain. This strain was deficient in OmpK35 and OmpK36; absence of the latter protein was due to the presence of IS1 in the ompK36 regulatory region. CONCLUSIONS: Resistance to selected beta-lactams in two clinical isolates of K. pneumoniae was due to interplay between the expression of OMPs and TEM-1.