Bacteriostasis ofEscherichia coliby milk: II. Effect of bicarbonate and transferrin on the activity of infant feeds

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Abstract
Fresh human and bovine milk are bacteriostaticin vitrofor only some (milk-sensitive) strains ofE. coli.The addition of bicarbonate to the test system potentiates the bacteriostasis so that otherwise milk-resistant strains are inhibited. By titration of the bicarbonate in the milk, it is possible to determine the minimum concentration that will activate milk against a milk-resistant strain but be ineffective in boiled milk, i.e. it potentiates a heat-labile system in milk and does not merely exert a direct toxic effect. This concentration is lower for human milk than for cows‘ milk and can be reduced even further by the addition of more iron-binding protein.Lactoferrin and bicarbonate may be present in the gut of the newborn. In an attempt to imitate conditions in the infant gut, we therefore reinvestigated,in vitroand in the presence of added bicarbonate and transferrin, the bacteriostatic activity againstE. coliof fresh breast-milk, commercial bottle-milk, and mixtures of these as fed to infants in this study. The results, and information about eventsin vivodeduced from the ratio of milk-sensitive to milk-resistant strains ofE. coliisolated from babies‘ stools, suggest that neonatal intestinal secretions may contribute to the bacteriostatic activity of their feeds so that (1) in fully breastfed babies all strains ofE. coliare inhibited to the same extent; there is no selection on the basis of milk sensitivity and equal numbers of strains resistant and sensitive to milk are found in the stools; (2) in fully bottle-fed babiesE. coliis not inhibited since the milk is non-bacteriostatic and again there is no selection; (3) in babies fed at the breast but bottle-milk supplemented, only milk-sensitive strains are inhibited; milk-resistant strains are not, and preferentially colonize the large intestines.

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