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Secretory IgA does not enhance the bacteriostatic effects of iron-binding or vitamin B12-binding proteins in human colostrum.

Human milk contains an unsaturated iron-binding protein (lactoferrin) and an unsaturated vitamin B12-binding protein. Lactoferrin has bacteriostatic properties, and a bacteriostatic role for the B12-binding protein has been postulated. In this study the bacteriostatic effect of lactoferrin was confirmed for strains of Escherichia coli, Pseudomonas and Proteus. Growth inhibition attributable to the unsaturated B12-binding protein could be demonstrated only with a known vitamin B12-dependent E. coli. It has previously been shown that the bacteriostatic effect of lactoferrin is potentiated by horse IgG antibody, and a similar potentiating effect of secretory IgA antibody in colostrum and milk would have obvious importance. An attempt was therefore made to demonstrate potentiation of bacteriostatic effects by naturally occurring secretory IgA antibody to E. coli. The results obtained indicate that secretory IgA antibody does not enhance the growth-inhibiting effects of either lactoferrin or the vitamin B12-binding protein.