Inhibition of Aerobacter cephalosporin beta-lactamase by penicillins.

Cephalosporinase (beta-lactamase) was obtained from cell washings of Aerobacter (Enterobacter) cloacae as a highly active preparation. An alkalimetric method was used to determine the enzyme activity and to estimate its inhibition by 6-amino-penicillanic acid derivatives. Their order of decreasing inhibitory effect was as follows: cloxacillin, oxacillin, methicillin, ampicillin, and penicillin G. We found that 2 to 3 ng of cloxacillin per ml was sufficient to decrease the enzyme activity by 50% in the presence of 400 mug of cephalosporin C per ml. Cloxacillin exerted a potentiating effect on the inhibition of the E. cloacae organisms by cephalosporin C.