Roles of beta-lactamases and porins is activities of carbapenems and cephalosporins against Klebsiella pneumoniae

Two clinical isolates of extended-spectrum beta-lactamase (ESBL)-producing Klebsiella pneumoniae were noted to be less susceptible than expected to im ipenem. Both were missing outer membrane proteins that serve as channels fo r antibiotic entry. The role of beta-lactamase in resistance was investigat ed by eliminating the original ESBL and introducing plasmids encoding vario us ESBLs and AmpC beta-lactamase types, by studying the effect of an increa sed inoculum, and by evaluating interactions with beta-lactamase inhibitors . The contribution of porin deficiency was investigated by restoring a func tional ompK36 gene on a plasmid. Plasmids encoding AmpC-type beta-lactamase s provided resistance to imipenem (up to 64 mu g/ml) and meropenem (up to 1 6 mu g/ml) in strains deficient in porins. Carbapenem resistance showed lit tle inoculum effect, was not affected by clavulanate but was blocked by BRL 42715, and was diminished if OmpK36 porin was restored, Plasmids encoding TEM- and SHV-type ESBLs conferred resistance to cefepime and cefpirome, as well as to earlier onyimino-beta-lactams. This resistance was magnified wit h an increased inoculum, was blocked by clavulanate, and was also lowered b y OmpK36 porin restoration. In addition, SHV-2 beta-lactamase had a small e ffect on carbapenem resistance (imipenem MIG, 4 mu g/ml, increasing to 16 m u g/ml with a higher inoculum) when porins were absent. In K. pneumoniae po rin loss can thus augment resistance provided either by TEM- or SHV-type ES BLs or by plasmid-mediated AmpC enzymes to include the latest oxyimino-beta -lactams and carbapenems.