The minimum inhibitory concentration of six antibiotics for ten clinical isolates of was determined. One of the isolates, which was unusually resistant to ampicillin and cephaloridine, possessed an intracellular β-lactamase. This enzyme, which was more active against cephaloridine than against the penicillins, was characterised. No β-lactamase activity could be detected in any of the moderately resistant strains, or in whole-cell preparations, or culture filtrates. The production of β-lactamase was not induced in resistant or sensitive strains by growth in media containing ampicillin.

The above evidence supports the view that strains of that are highly resistant to β-lactam antibiotics owe this property to the presence of an intracellular β-lactamase.

Attempts to transfer antibiotic resistance from and vice versa, and from one strain of to another, were unsuccessful.


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