Lactobacilli and bifidobacteria in human breast milk: influence of antibiotherapy and other host and clinical factors.

Bacterial gut colonization in early life is a process that exerts a short-, medium-, and long-term influence on the health status of a host, and that involves bacteria arising from different sources (1); among them, culture-dependent studies have revealed that human milk is a source of live staphylococci, streptococci, lactic acid bacteria, bifidobacteria, propionibacteria, corynebacteria, and closely related Gram-positive bacteria to the infant gut (2). Several studies have shown that there is a mother-to-infant transfer of bacterial strains belonging, at least, to the genera Lactobacillus, Staphylococcus, Enterococcus, and Bifidobacterium through breast-feeding (3–7). In fact, human milk constitutes 1 of the main sources of bacteria to the breast-fed infant gut since a baby consuming approximately 800 mL/day of milk would ingest between 1 × 105 and 1 × 107 bacteria daily (8). It has been suggested that exposure of the breast-fed infant to such a wealth of bacterial phylotypes through breast-feeding may exert beneficial effects against several diseases (9). Breast-feeding has been shown to improve infant health outcomes lowering the risk of respiratory and gastrointestinal tract infections, necrotizing enterocolitis, otitis media, and allergic disease and to prevent later health problems such as inflammatory bowel disease, obesity, and type 2 diabetes mellitus (10). The application of culture-independent molecular techniques, and particularly those based on 16S rRNA genes, allowed a complementary biodiversity assessment of the human milk microbiome. The use of such techniques confirmed the dominance of staphylococci and streptococci, the relatively frequent presence of lactic acid bacteria and bifidobacteria, and the existence of DNA belonging to other bacterial groups, such as some Gram-negative bacteria (5,11–13). Recently, the first microbiome study focused on human milk was published and the results indicated that milk bacterial communities were generally complex (9). Among the hundreds of operational taxonomic units detected in the milk of every woman, only 9 (Streptococcus, Staphylococcus, Serratia, Pseudomonas, Corynebacterium, Ralstonia, Propionibacterium, Sphingomonas, and Bradyrhizobiaceae) were present in every sample from every woman. On the contrary, milk bacterial community was generally stable over time within an individual (9). In contrast to staphylococci, streptococci, corynebacteria, or propionibacteria, which seem to be widespread in human milk, the presence of lactobacilli and bifidobacteria seems to be more variable among women (9,11,14,15). Such variability may be the consequence of isolation difficulties, owing to fastidious growth and incubation requirements, or may be the result of the technical bias associated to molecular studies. It could also be because of host peculiarities; it has been suggested that the human milk microbiome is influenced by several factors that significantly skew its composition (16). In this context, the objective of this study was to assess whether demographic or clinical factors, such as country and date of birth, infant age, delivery mode, or antibiotherapy during pregnancy and lactation, may exert an influence on the bifidobacterial and lactobacillic population present in the breast milk of healthy women.