476. Observations on the influence of inhibitory substances produced by the lactobacilli of Gruyère cheese on the development of anaerobic spore-formers

1. The failure of clostridia to produce a butyric acid fermentation, when known to be present in Swiss-type cheese, is discussed. 2. It is suggested that inhibitory substances produced by the predominating lactic flora of young cheese are partly responsible for the suppression of the fermentation. 3. Details are given of the techniques used to investigate this supposition. 4. The evidence was obtained by examining fifty-one samples of Gruyère cheese, 78% of which were contaminated with clostridia. 85% of the samples contained inhibitory lactobacilli. 5. In 30% of the samples the inhibitory phenomena were sufficiently marked to give clear zones of inhibition against Staphylococcus aureus when a suspension of cheese was placed in penicillin cups. 6. Inhibition of Clostridium butyricum was demonstrated with pure cultures of lactobacilli. The cultures could grow at temperatures from 30 to 50° C, but were inhibitory only at certain temperatures of incubation which appear to be a strain characteristic. 7. The inhibitory substance(s) concerned is not nisin because it is acid and heat labile.