Differentiation of resistance determinants to cefamandole and cefoxitin in Enterobacter cloacae strains.

The mechanism of resistance to cefamandole and cefoxitin was investigated in Enterobacter cloacae with the aid of cefamandole-resistant variants and their derived beta-lactamase-deficient mutants. Cefamandole-resistant variants were easily obtained from clinical isolates by direct selection. Massive beta-lactamase production seemed to be the underlying resistance mechanism, although lack of penetrability may further substantiate this resistance. The mechanism of resistance to cefoxitin in parent, variant and mutant strains on the other hand was more complex, and probably due to a complex interrelation of parameters. Apart from relative instability to beta-lactamases and lack of penetrability, the high beta-lactamase-inducing power of cefoxitin is perhaps the most important determinant in the resistance of Enterobacter to this compound.