[Level of "internal" resistance of E. coli cultures to benyzlpenicillin]

Interaction of benzylpenicillin inactivation by beta-lactamase and the growth of E. coli in the presence of various antibiotic concentrations was studied. It was found that penicillin inactivation began immediately after addition of the culture to the medium in spite of the fact that the number of the viable cells at that time decreased. The culture began to multiply only when benzylpenicillin concentration in the medium lowered up to a definite level (6-7 gamma/ml). Such a concentration must determine the level of the "inner" culture resistance to benzylpenicillin, not connected with the beta-lactamase effect. Moreover, the "inner" resistance proved to be the same for the cultures of E. coli, markedly differing by the amounts of beta-lactamase produced and the resistance levels to benzylpenicillin. Therefore, the routine method for determination of the "inner" resistance of the beta-lactamase-producing cultures to penicillins, using low innoculation doses produced higher results as compared to the real "inner" resistance. Significant differences observed between the values of the "inner" resistance (6-7 gamma/ml) and the MIC of benzylpenicillin at high innoculation doses with respect to E. coli 2 (129 gamma/ml) and E. coli 82 (15000 gamma/ml) may probably be explained by destruction of the drug by beta-lactamase.