A microcalorimetric study on the action of penicillins on Staphylococcus aureus and Escherichia coli.

The heat generated by cultures of a strain of Escherichia coli and of a beta-lactamase- and a non-beta-lactamase-producing strain of Staphylococcus aureus were registered by a flow microcalorimeter of the heat conduction type. When exposing the non-beta-lactamase-producing strain to benzylpenicillin, an effect that appeared to correspond to the 'paradoxical zone phenomenon' was found, i.e. the inhibitory effect on the bacterial metabolism, as judged from the heat generated, was smaller when using 0.1-0.2 microgram ml-1 of benzyl-penicillin (5 and 10 x minimum inhibitory concentration, MIC) than with 0.04 microgram ml-1 (2 x MIC) and 2.0-200 microgram ml-1 (100-10,000 x MIC). Similar paradoxical response were found when the effects of ampicillin on E. coli and on the non-beta-lactamase-producing strain of S. aureus were studied. In the beta-lactamase-producing strain, no decrease in the heat was obtained when benzylpenicillin was used in concentrations up to 5.0 microgram ml-1 (10 x MIC), whereas a drug concentration of 50 microgram ml-1 (100 x MIC) caused a slight reduction. The addition of dicloxacillin in concentrations of 0.25-12.5 microgram ml-1 (2-100 x MIC) caused a concentration-dependent reduction in the heat developed.