Bacteriostasis of a milk-sensitive strain of Escherichia coli by immunoglobulins and iron-binding proteins in association

Abstract

The growth of a milk-sensitive strain of Escherichia coli in 1% peptone water can be inhibited for at least 3 h by IgA isolated from human milk or IgG₁ from bovine colostrum acting with native iron-binding proteins from milk or serum. The immunoglobulins alone are inactive; the native iron-binding proteins alone are sometimes partially active. All this activity is inconsistent and not always enhanced by the addition of bicarbonate ions.

The growth of E. coli in human milk that has been inactivated by heating at 100° is consistently inhibited by IgA or IgG₁ acting with native iron-binding proteins. The immunoglobulins are inactive alone but the iron-binding proteins have considerably more activity when added alone to inactivated milk than to peptone water, suggesting that the growth medium is contributing to or stabilising the activity. The addition of bicarbonate ions is without effect. Attempted absorption of antibody with suspensions of E. coli and replacement of bacteriostatic activity by addition of purified milk proteins has not, however, suggested any participants in the bacteriostasis of milk-sensitive strains other than antibody and iron-binding protein.

Bacteriostasis is abolished by saturating the transferrins with iron. The iron-free apo-derivatives are not more inhibitory than the native proteins except for human apo-lactotransferrin in peptone water which inhibits growth completely. This latter inhibition is not attributable to the low pH and 10–100 times more iron is needed to abolish this activity than is needed to abolish that of bovine apo-lactotransferrin.