The effect of unsaturated fatty acids on Lactobacillus helveticus and other Gram-positive micro-organisms

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Abstract
Addition of 160 [gamma] of stearic acid or palmitic acid to 10 ml. of the medium (Snell and Strong, Ind. Eng. Chem. 11, 346, 1939, as modified by Barton-Wright and Booth, Biochem. Jour. 37, 25, 1943), caused an increase in acid production during the first 24 hrs. incubation. Caprylic and caproic acids showed little or no effect. Similar results were obtained with each of these four acids on the same medium freed from lipids by extracting twice with CHCl3. Oleic acid was without effect in the unextracted medium, but in the extracted medium 160 [gamma]/10 ml. completely inhibited growth and acid production during 24 hrs. and partially inhibited them during 48 hrs.; after 72 hrs. the acid production was almost the same as in control samples without oleic acid. Addition of 160 [gamma]/10 ml. of linolic acid completely inhibited growth for 72 hr. in both unextracted and extracted media. Linolenic acid behaves similarly. The inhibitory action of these acids was reversed by the addition of surface acting compounds such as lecithin or cholesterol. The author suggests that in the microbiological assay of riboflavin, lipids should be removed from the medium and from the material under test by extraction with CHCl3, and that the tubes should be incubated for 72 hrs. Linolenic acid inhibits other Gram-positive bacteria but not the Gram-negative Proteus vulgaris and Escherichia coli.