DIFFERENCES IN THE RESISTANT VARIANTS OF ENTEROBACTER-CLOACAE SELECTED BY EXTENDED-SPECTRUM CEPHALOSPORINS

The rates of development of resistance to ceftriaxone, ceftazidime, ce fepime, and cefpirome in 10 strains of Enterobacter cloacae were deter mined by daily transfer for 7 days to fresh medium containing twofold serial dilutions of the antibiotics. Development of resistance to ceft riaxone was the most rapid; this was followed by ceftazidime, cefpirom e, and cefepime. Resistant variants selected by ceftriaxone and ceftaz idime were cross-resistant and produced very high levels of beta-lacta mase. On the other hand, resistant variants selected by cefepime and c efpirome often had moderately high levels of beta-lactamase and dimini shed levels of the 39- to 40-kDa porin protein.