The growth of Clostridium welchii type A NCTC 8246 was inhibited by the contents of the small intestine of pigs from which food had been withheld for 24 hr ; no inhibitory activity was observed by the contents of the stomach, caccum and large intestine. Inhibitory activity was absent from the contents of the small intestine during the early stages of digestion but first appeared about 6 hr after the pigs had been given food. This indicated that under normal feeding practice there were periods during each day when the contents of the small intestine would be inhibitory for C. welchii. The inhibitory activity was in the lipid fraction of the intestinal contents. Fractionation of the intestinal lipids showed that the inhibitory substances were linoleic and arachidonic acids, lysolecithin and an unidentified phospholipid. The poly-unsaturated fatty acids and lysolecithin appeared to be derived mainly from the action of pancreatic phospholipase A on biliary lecithin in the lumen of the small intestine. The possibility that these inhibitory lipids control to some extent the numbers of C. welchii in the alimentary tract of the pig is discussed.
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