Cephalosporin resistance in strains of Klebsiella oxytoca isolated during antibiotic therapy.

In the course of multiple antibiotic treatment of a seriously ill patient, a high level of resistance to cefamandole, cefuroxime and ceftriaxone was found among consecutive isolates of Klebsiella oxytoca, other species isolated remaining susceptible. Susceptibility to cefotaxime was less affected, and the activity of cefoxitin and moxalactam was unchanged. Resistant K. oxytoca strains synthesized a large amount of a broad-spectrum beta-lactamase, able to hydrolyze ceftriaxone, cefuroxime and cefotaxime. The enzyme level as well as the ability to hydrolyze different cephalosporins with different rates explained the in vitro susceptibilities observed reasonably well. The initially susceptible K. oxytoca strain easily converted in vitro to variants producing a high level of beta-lactamase after exposure to ceftriaxone, without concomitant acquisition of high-level resistance to this antibiotic, suggesting that additional and at present unknown factors also contributed to resistance of the clinical isolates.