The antimicrobial function of milk lipids.

Milk lipids serve not only as nutrients but as antimicrobial agents that constitute a defense system against microbial infections that occur at mucosal surfaces. The lipid fraction of milk develops antimicrobial activity in the gastrointestinal tract of suckling neonates as a result of lipolytic activity which converts milk triglycerides to antimicrobial fatty acids and monoglycerides. Antimicrobial milk lipids may be particularly important in protecting infants with an inadequate secretory immune response from infection. The lipid-dependent antimicrobial activity of milk is due to medium-chain saturated and long-chain unsaturated fatty acids and their respective monoglycerides released by lipases in the gastrointestinal tract. The antimicrobial activity of fatty acids and monoglycerides is additive and consequently it is their combined concentration that determines the lipid-dependent antimicrobial activity of milk. Microbial inactivation occurs rapidly by membrane destabilization. The antimicrobial activity of milk lipids can be duplicated using purified fatty acids and monoglycerides. It should be possible, therefore, to supplement banked human milk to provide lipid-dependent antimicrobial activity from the moment of ingestion (Schanler et al., 1986). This could reduce the risk of viral transmission from mother to infant through milk. Milk lipids also could be adapted for use at mucosal surfaces other than those in the gastrointestinal tract to reduce vertical transmission of pathogens during birth.