Outer membrane protein profiles of clonally related Klebsiella pneumoniae isolates that differ in cefoxitin resistance

Eleven genotypically related Klebsiella pneumoniae isolates were obtained from 11 patients. All isolates were resistant to third-generation cephalosporins due to the production of SHV-2a extended-spectrum β-lactamase. Comparison of the outer membrane protein profiles revealed one isolate that lacked porins. This porin-deficient isolate was also resistant to cefoxitin (MIC 128 μg ml−1) and moxalactam (MIC 64 μg ml−1) and had elevated MIC of meropenem (2 μg ml−1) when compared to porin-expressing isolates (2-8, 4 and <0.06-0.125 μg ml−1, respectively). Higher MICs, associated with loss of porins in outer membrane, were also observed for cefotaxime (4-8-fold), cefepime (>2-16-fold), ciprofloxacin (4-16-fold), imipenem and aztreonam (2-16-fold), but there was no significant difference among MICs of ceftazidime. The porin-deficient mutant was probably selected in vivo during ofloxacin therapy.