Resistance to cephalosporins among gentamicin-resistant klebsiellae

The susceptibilities to amikacin, netilmicin, a variety of cephalosporins and cephamycins, and monobactam (SQ 26776) were determined for a series of gentamicin- and multiply-resistant klebsiellae (33 different serotypes and 28 serologically non-typable strains). All showed transferable resistance to netilmicin (MICs 16 – > 100 mg/l). Mode MICs of amikacin, cefuroxime and cefoxitin were 4 mg/l. Cefotaxime, ceftriaxone, cefotetan, moxalactam, ceftazidime and monobactam all had higher intrinsic activity (mode MIC 0·5 mg/l). Eight isolates (mostly Klebsiella oxytoca) were insensitive to cefuroxime. Most of them were also resistant to cefotaxime, ceifriaxone and monobactam but all were highly sensitive to ceftazidime and cefotetan. The resistance was due to four different chromosomally mediated broad spectrum β-Iactamase enzymes, only one of which has been described previously. One of these enzymes also had weak activity against moxalactam but the single isolate producing it was only relatively insensitive to that agent (MIC 10 mg/l).