Microbiological Aspects of Penicillin

Abstract

Contrary to results with the plate method, by turbidimetric measurements the same amount of penicillin is required to effect a 50% growth inhibition of Staphylococcus aureus H in nutrient broth at pH 5, 5.5, 6, 6.5, 7, 7.5 and 8. The presence of glucose and other sugars in the medium causes irregular breaks in the inhibition curves. This is apparently due to an alteration or partial inactivation of penicillin which occurs when sterile penicillin and glucose broth solutions are allowed to stand before inoculation. The effect is not obtained in the absence of glucose. It also takes place during the regular 16-hr. incubation period. The inactivation is less at 2[degree] than at 37[degree]C and could be found only at certain conc, levels of penicillin. Penicillin inhibition of S. aureus H is due to logarithmic prolongation of generation time and not to selection of resistant cells. Small losses occur during pasteurization of penicillin solns. The stability of penicillin is best between 4.8 and 7.9. It is inactivated very rapidly beyond these -extremes. A short-time (3- to 5-hr.) turbidimetric assay employing Bacillus adhaerans as test organism and accelerating growth by forced aeration (shaking) is described.