499. The inhibition of micro-organisms by raw milk: I. The occurrence of inhibitory and stimulatory phenomena. Methods of estimation

Abstract

1. A diffusion cup technique using Micrococcus lysodeikticus has been found suitable for lysozyme estimation. By this method raw milk gave only small zones of inhibition. The inhibitory substance of milk was shown not to be lysozyme.2. Dose-response curves (dilution v. pH) were obtained with Streptococcus pyogenes as the test organism. They indicated that the effect of milk on this organism was the result of the interaction of one stimulatory and at least two inhibitory substances.3. The existence of two inhibitory substances was confirmed by dilution assays, one, L₁, occurring mainly in colostrum, the other, L₂, occurring mainly in milk. The inhibition is greatest when L₁ and L₂ act simultaneously.4. The pH and heat stabilities of the two substances are slightly different. L₁ is most stable at pH 6−6·5, 90% being destroyed at 68° C. L₂ is most stable at pH 7 and 95% is destroyed at 74°C.