Iron-binding proteins and other factors in milk responsible for resistance to Escherichia coli

Human milk contains large amounts of the iron-binding protein lactoferrin. This is normally unsaturated with iron. It also contains large amounts of IgA and small amounts of IgG and IgM. A combination of lactoferrin and specific antibody has a powerful bacteriostatic effect on Escherichia coli. In sucking infants the milk proteins probably reach the small intestine intact. Experiments with sucking guinea pigs show that milk suppresses E. coli in the gut and that the unsaturated iron-binding protein plays an essential role in the bacteriostatic reaction. Inhibited E. coli appear to be acutely iron deficient. E. coli growing slowly in iron-deficient media show abnormal forms of certain aminoacyl tRNAs. In bacteria inhibited by colostrum the proportion of abnormal tRNA is as high as 90%. These abnormal tRNAs are converted to the normal form by the addition of iron. This occurs in the absence of further RNA synthesis and is accompanied by renewed bacterial growth. The normal flora of the gut also plays an important role in resistance. Human milk has a low buffering capacity and bacterial fermentation of lactose produces a low pH.e. coli is inhibited by acetic acid/acetate buffer at pH 4.8-5.6, whereas these conditions allow normal growth of Lactobacillus bifidus. The faeces of babies fed on breast milk have a low pH, low counts of E. coli and high counts of L. bifidus. Artificially fed babies have more alkaline faeces which contain few L. bifidus and large numbers of E. coli.