Nonimmunoglobulin Fraction of Human Milk Inhibits the Adherence of Certain Enterotoxigenic Escherichia coli Strains to Guinea Pig Intestinal Tract

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Abstract
The protecting effect of human milk against intestinal infections has been well documented, but its mechanism not completely understood. We have examined the effect of the nonimmunoglobulin fraction (NIgF) of human milk and colostrum on bacterial adherence to the intestinal tract. The NIgF was prepared by passing the milk through an imimmosorbent column containing rabbit antihuman γ-globulin (IgG and IgA). The effluent fraction did not contain γ-globulins as shown by immunodiffusion on agarose and by using rabbit antihuman Ig, that was then detected with fluorescently-labeled goat antirabbit Ig. The effect of the NIgF of human milk on the adherence of enterotoxigenic Escherichia coli strains to guinea pig intestinal tract was quantitatively determined using radiolabeled bacteria which were incubated with suspensions of viable intestinal cells. Thirteen to 17 bacteria adhered per intestinal cell. NIgF of human milk and colostrum (300 μl, 6.7mg) caused about 50% inhibition of the adherence of enterotoxigenic E. coli strains whose attachment was mediated by colonization factor antigen I and II. No inhibition was noted on the adherence of enterotoxigenic E. coli strains containing type I pili. The inhibitory activity resisted boiling and proteolytic digestion with trypsin, but was completely abolished by periodate treatment, indicating that carbohydrate residues were probably involved. Examination of the effect of NIgF of human milk on bacterial adherence to intact intestinal surfaces revealed comparable results. Observations with scanning electron microscopy confirmed, morphologically, the attachment of the bacteria and the inhibitory effect of human milk. It is concluded that receptor-like glycocompounds in human milk and colostrum inhibit the adherence of certain enterotoxigenic E. coli strains to the intestinal mucosa. This may play a role in protecting infants against intestinal infections.