Infectious resistance to carbenicillin in Proteus rettgeri: occurrence of two different penicillinases in the same strain.

Sir: Ampicillin, carbenicillin and cephalosporins are beta-lactam antibiotics effective against some gram-negative bacteria. However, they may be neutralized by enzymes, usually beta-lactamases. Different betalactamases have been found among enterobacteria1^ Although ampicillin and carbenicillin are closely related, cross resistance does not always exist. The occurrence of a segregation in the resistance to carbenicillin in a Proteus rettgeri population led us to isolate three strains supposed to be three variants of the same strain. These variants LG21, LG22 and LG23 show the following minimal inhibitory concentrations (M.I.C.) (Table 1).