In vivo selection of OmpK35-deficient mutant after cefuroxime therapy for primary liver abscess caused by Klebsiella pneumoniae

Objectives: The aim of the study was to characterize the genetic basis of β-lactam resistance developed in clinical isolates of Klebsiella pneumoniae after exposure to cefuroxime. Methods: Clinical features of two episodes of liver abscess caused by K. pneumoniae in a diabetic patient were reported. Four isolates (KP₁/KP₂ and KP₃/KP₄) of K. pneumoniae were recovered from cultures of blood/pus in the first and second episodes, respectively. Laboratory investigation of the K. pneumoniae isolates included genotyping by PFGE, resistance gene analysis by PCR amplification and DNA sequencing, and outer membrane protein analysis by SDS-PAGE. Results: KP₃ and KP₄ were recovered after a 21 day cefuroxime therapy and demonstrated identical genotypes to that of KP₁ and KP₂. However, compared with KP₁ and KP₂, emerging resistance to piperacillin, cefalotin, cefuroxime and cefoxitin was observed. The other antibiotics tested, except ampicillin, retained the same effectiveness against the four isolates, although increases (4- to 8-fold) in the MICs of cefotaxime, ceftriaxone, ceftazidime, cefepime, flomoxef and aztreonam were observed in KP₃ and KP₄. None of the isolates produced extended-spectrum β-lactamases or plasmid-mediated AmpC β-lactamases. Deficiency in the expression of an outer membrane protein (OmpK35) was observed in the cefuroxime-resistant isolates, KP₃ and KP₄. Conclusions: The increased resistance to cephalosporins in these clinical isolates of K. pneumoniae after exposure to cefuroxime might be related to the loss of OmpK35.