ObjectivesCertain milk factors may help to promote the growth of a host-friendly colonic microflora (e.g. bifidobacteria, lactobacilli) and explain why breast-fed infants experience fewer and milder intestinal infections than those who are formula-fed. The effects of supplementation of formula with two such milk factors was investigated in this study.Materials and MethodsInfant rhesus macaques were breast-fed, fed control formula, or formula supplemented with glycomacropeptide (GMP) or &agr;-lactalbumin (&agr;-LA) from birth to 5 months of age. Blood was drawn monthly and rectal swabs were collected weekly. At 4.5 months of age, 108colony-forming units of enteropathogenicE.coliO127, strain 2349/68 (EPEC) was given orally and the response to infection assessed. The bacteriology of rectal swabs pre- and post-infection was determined by culture independent fluorescence in situ hybridization.ResultsPost-challenge, breast-fed infants and infants fed &agr;-LA-supplemented formula had no diarrhea, whilst those infants fed GMP-supplemented formula had intermittent diarrhea. In infants fed control formula the diarrhea was acute.ConclusionsSupplementation of infant formula with appropriate milk proteins may be useful for improving the infant's ability to resist acute infection caused byE.coli.