Differences in inhibition of the growth of commensal and enteropathogenic strains of Escherichia coli by lactotransferrin and secretory immunoglobulin A isolated from human milk.

Immunoglobulins from bovine and human colostrum and milk and lactotransferrin (LTF) from human milk were investigated for bacteriostatic activity against Escherichia coli growing in a tissue culture medium. When tested separately, LTF or secretory immunoglobulin A (sIgA) from pooled human milk showed only slight bacteriostatic activity against human commensal or enteropathogenic strains of E. coli. Together, they had a considerable bacteriostatic effect, but only against strains of enteropathogenic serotype. This activity of the sIgA from pooled human milk was consistent for all enteropathogenic serotypes tested, but sIgA isolated from individual milk samples was inactive against some serotypes, and this specificity was associated with antibody to the O antigens. The activity of the sIgA was stable to heat at 56 degrees for 2 h but was lost progressively on heating at 65 degrees for 10 min or longer. Bovine colostral IgGl was without bacteriostatic effect alone. Together with LTF, it was active against a strain pathogenic to calves but not against human enteropathogenic strains. Tests on rabbit antisera raised against commensal enteropathogenic strains of E. coli showed that for the enteropathogens the bacteriostatic activity (in association with LTF) was high and was specific for the serotype of the eliciting strain, but bacteriostatic activity was low or absent in the antisera to commensal strains in spite of the presence of high titres of agglutinating antibodies to these strains.