Penicillinase and Ampicillin Resistance in a Strain of Escherichia coli

SUMMARY: A penicillinase-positive variant was isolated from a penicillinase-negative strain of Escherichia coli 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 E. coli thus differs qualitatively and quantitatively from other penicillinases.