Beta-Lactamase Activity in Shigella sonnei

The basis of ampicillin resistance in Shigella sonnei was analyzed for a number of isolates. Most isolates that were resistant to ampicillin and cephalothin failed to transfer resistance. Isolates resistant to cephalothin and ampicillin were inhibited by mecillinam, cefamandole, and cefoxitin. Six strains were selected for study. These differed on the basis of inhibition by 10 beta-lactam antibiotics and on differences in beta-lactamase activity. There was no correlation between the inhibitory concentration of antibiotic and the amount of antibiotic hydrolyzed. Use of sodium chloride and cloxacillin inhibition and acrylamide electrophoresis demonstrated that ampicillin resistance in Shigella is mediated by a number of different beta-lactamases. Organisms can contain more than one type of beta-lactamase.