Selection during cefepime treatment of a new cephalosporinase variant with extended-spectrum resistance to cefepime in an Enterobacter aerogenes clinical isolate.

Enterobacter aerogenes resistant to cefepime (MIC, 32 microg/ml) was isolated from a patient treated with cefepime for an infection caused by a strain of E. aerogenes overproducing its AmpC beta-lactamase (MIC of cefepime, 0.5 microg/ml). The AmpC beta-lactamase of the resistant strain had an L-293-P amino acid substitution and a high k(cat)/K(m) ratio for cefepime. Both of these modifications were necessary for resistance to cefepime.