From the Human Milk Microbiota to the Human Milk Metagenome: Evolution of Methods to Study Human Milk Microbial Communities

Traditionally, human milkwas considered to be contaminated with bacteria from the skin and nipples. Studies on the microbiology of human milkrestricted to the identification of potential pathogenic bacteria in stored milkof milkbanks showed that coagulase-negative staphylococci and streptococci were normal inhabitants of human milk. Later, the inclusion of a higher number of different culture media allowed the identification of other bacteria like lactic acid bacteria, propionibacteria, and occasionally gram-negative bacteria. The application of culture-independent molecular techniques, and particularly those based on 16S rRNA genes, has provided a complementary biodiversity assessment of the human milkmicrobiome. Molecular techniques have revealed the presence of DNA belonging to obligate anaerobicbacterial taxa. That fact supports the hypothesis that the presence of bacteria in human milkis not mere contamination and that an endogenous route may exist. Thus, human milkhas its own complex microbiota that can be affected by a woman’s health as well as predelivery and postdelivery factors.