Bacteriostasis of Escherichia coli by milk. III. The activity and stability of early, transitional and mature human milk collected locally.

Milk from 150 local mothers has been assayed for bacteriostatic activity for milk-sensitive and milk-resistant indicator strains of Escherichia coli. Activity is greatest in colostrum which is active directly against all strains of E. coli. One week after delivery of the baby, milk is active against the milk-sensitive strain and becomes active against the milk-resistant strain in the presence of physiological amounts of bicarbonate and iron-binding protein. This activity decreases within 2--4 days on keeping milk unheated at 4 degrees C but is preserved for at least 4 months and often up to 2 years in milk heated to 56 degrees C then stored at 4 degrees C or in milk frozen, unheated, at -28 degrees C provided it is not repeatedly thawed and frozen. Later lactation milks are usually indistinguishable in activity from 1-week post-partum milk but may be less stable on storage particularly if frozen. Lyophyilization in vacuo preserves activity of early-lactation milk for at least 6 months. Heating milk to above 65 degrees C causes a progressive loss of activity which can be partially restored by adding bicarbonate and iron-binding protein. Iron abolishes the activity of milk and reduces that of colostrum.