SUMMARY: A penicillinase-positive variant was isolated from a penicillinase-negative strain of during ‘training’ towards ampicillin resistance. The penicillinase activity of the variant was increased tenfold when the organisms were disrupted in a Milner press or treated with toluene, suggesting the presence of a permeability barrier. The enzyme released by disrupting the organisms was not precipitated by centrifugation nor removed by filtration. The penicillinase activity of toluene-treated organisms was easily removed by centrifugation. Disruption of the organisms released enzyme into solution whereas it remained intracellular in toluene-treated organisms. The variant strain, although resistant to over 200 μg. ampicillin/ml., did not appreciably destroy it; of other penicillins tested only penicillins G and V were readily destroyed. The enzyme was a ‘penicillinase’ (β-lactamase) since the product was penicilloic acid. The enzyme was not inducible: exposure of organisms to 6-amino-penicillanic acid or phenoxybenzylpenicillin produced a threefold increase in penicillinase activity not due to true induction but to an increased permeability. The penicillinase of this strain of thus differs qualitatively and quantitatively from other penicillinases.


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