Strains of enterobacteria isolated in a cardio-thoracic hospital and a rural general practice were collected for three years. The organisms were tested for sensitivity to four βlactam antibiotics, and the βlactamases from resistant strains were subsequently studied. There was no evidence of single-source infection with most of the strains.

Investigations were carried out on 1050 of the isolates. Eighty per cent, of the and cultures were sensitive to the two penicillins and two cephalosporins used and were omitted from further studies. The organisms resistant to 100 μg per ml of at least one of the four drugs were grouped taxonomically and the β-lactamases from 325 of them were examined. Relationships between the antibiotic resistance of the organisms and the substrate specificities and inhibitor profiles of their enzymes were studied. Seven β-lactam antibiotics were used to determine enzyme substrate-specificities and four for the inhibitor profiles. and were predominantly resistant to the cephalosporins and produced cephalosporinases that were inactivated by all the inhibitor compounds. and were mainly penicillin resistant. Klebsiella enzymes attacked penicillins and cephalosporins, but were not inhibited. The β-lactamases from strains showed a wide diversity of substrate degradation and inhibitor profiles, partly because of the frequency with which the enzyme activity was R-factor-mediated.

Most of the previously reported β-lactamases from enterobacteria were encountered among our organisms. The resistance patterns and enzyme types of the strains were well correlated with the generic groups of the producer organisms.


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