Abstract

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 µg/ml and those inhibited by 32–500 µg 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 β-lactam antibiotics have been detected among enterobacteria.

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