Bacteriostatic effect of human milk and bovine colostrum on Escherichia coli: importance of bicarbonate

Abstract

At pH 7.4 and in the presence of NaHCO3, human milk and bovine colostrum inhibited the growth of E. coli O111. Adding sufficient Fe to saturate the Fe binding capacity of the lactoferrin present in the milk or colostrum prevented bacteriostasis. At pH 6.8 neither milk nor colostrum inhibited E. coli O111. Adjusting the pH to 7.4 with NaHCO3 resulted in the development of bacteriostatic activity. Adjusting the pH to 7.4 with NaOH was ineffective. Dialyzed colostrum and milk inhibited bacterial growth at pH 6.8 in the absence of added NaHCO3; addition of citrate or Fe abolished bacteriostasis. The chromatographic elution profile of tyrosyl-tRNA from Fe-replete E. coli differs significantly from that of tyrosyl-tRNA from Fe-deficient organisms. Examination of the elution profile of tyrosyl-tRNA from E. coli O111 growing in colostrum without added NaHCO3 showed that such bacteria were fully replete in Fe. The nature of the elution profile of tyrosyl-tRNA also showed that Fe was freely available to the bacteria when citrate was added to dialyzed colostum but not available in its absence, even at pH 6.8. The bacteriostatic action of milk and colostrum, due to the combined action of antibody and lactoferrin, apparently depends on the addition of bicarbonate to counteract the Fe-mobilizing effect of the citrate normally present in these secretions.