Molecular genetics of resistance to both ceftazidime and beta-lactam-beta-lactamase inhibitor combinations in Klebsiella pneumoniae and in vivo response to beta-lactam therapy.

The molecular basis of ceftazidime resistance in 2 isolates of Klebsiella pneumoniae was studied. The first (21300) expressed resistance to ceftazidime and piperacillin-tazobactam. The second (26139) expressed resistance to ceftazidime but remained susceptible to piperacillin-tazobactam. The 2 strains harbored similar large plasmids that hybridized to TEM- and SHV-related beta-lactamase genes. An Escherichia coli strain harboring the plasmid conferring resistance to both compounds (pLRM7) produced beta-lactamases of pI 5.9 (TEM-6) and pI 7.6 (SHV-1). E. coli harboring the other plasmid (pLRM8) expressed only the TEM enzyme because of insertion of IS15 within blaSHV-1. In vivo studies suggested that resistance to beta-lactam-beta-lactamase inhibitor combinations conferred by pLRM7 will be clinically important. Clinical resistance to both extended-spectrum cephalosporins and beta-lactam-beta-lactamase inhibitor combinations is achievable via the production of two enzymes, with only one possessing an extended spectrum of activity.