Resistance categories of enterobacteria to beta-lactam antibiotics.

The activities of cephaloridine, cephalothin, and ampicillin against dense cultures of 103 ampicillin-resistant gram-negative bacilli were compared in a turbidimetric system. The organisms were divided into two groups according to their behavior in conventional minimal inhibitory concentration titrations: those fully resistant to ampicillin at a concentration of 500 mug/ml and those inhibited by 32-500 mug of ampicillin/ml. Of the 83 strains in the first group, most were more sensitive to cephalothin than to cephaloridine, whereas the 20 strains in the second group exhibited a number of interesting properties peculiar to them. On the basis of these and previous results, six distinct categories of resistance to beta-lactam antibiotics have been detected among enterobacteria.