Invited Review
The human milk microbiota: Origin and potential roles in health and disease

https://doi.org/10.1016/j.phrs.2012.09.001Get rights and content

Abstract

Human milk has been traditionally considered sterile; however, recent studies have shown that it represents a continuous supply of commensal, mutualistic and/or potentially probiotic bacteria to the infant gut. Culture-dependent and -independent techniques have revealed the dominance of staphylococci, streptococci, lactic acid bacteria and bifidobacteria in this biological fluid, and their role on the colonization of the infant gut. These bacteria could protect the infant against infections and contribute to the maturation of the immune system, among other functions. Different studies suggest that some bacteria present in the maternal gut could reach the mammary gland during late pregnancy and lactation through a mechanism involving gut monocytes. Thus, modulation of maternal gut microbiota during pregnancy and lactation could have a direct effect on infant health. On the other hand, mammary dysbiosis may lead to mastitis, a condition that represents the first medical cause for undesired weaning. Selected strains isolated from breast milk can be good candidates for use as probiotics. In this review, their potential uses for the treatment of mastitis and to inhibit mother-to-infant transfer of HIV are discussed.

Section snippets

Bacterial diversity of human milk

Human milk is a complex species-specific biological fluid adapted to satisfy the nutritional requirements of the rapidly growing infant; additionally, it educates the infant immune system and confers a certain degree of protection against pathogens [1]. These effects reflect the synergistic action of many bioactive molecules, present in colostrum and milk, including immunocompetent cells, immunoglobulins, fatty acids, polyamines, oligosaccharides, lysozyme, lactoferrin and other glycoproteins,

Functions of human milk bacteria in the infant gut

In the last years, some studies have shown that human milk bacteria may play several roles in the infant gut. First of all, they can contribute to the reduction of the incidence and severity of infections in the breastfed infant by different mechanisms, such as competitive exclusion [35], production of antimicrobial compounds [6], [9], [10], [35], or improvement of the intestinal barrier function by increasing mucine production and reducing intestinal permeability [35]. Recently, the

Origin of the bacteria isolated from breast milk: is there a bacterial entero-mammary pathway?

The origin of the bacteria present in breast milk has become a controversial issue in the last years. Traditionally, it was believed that milk harbored bacteria that were just the result of contamination with bacteria from the mother's skin or the infant's oral cavity [19].

Infrared photography [49] has shown that a certain degree of retrograde flow back into the mammary ducts can occur during suckling. Obviously, such back flow may provide an ideal route for the exchange of bacteria from the

Human milk bacteria as biotherapeutic agents

In recent years, the problems associated to the spread of clinical antibiotic resistances among pathogenic bacteria and to the rise of allergic or inflammatory diseases in developed countries have led to a new interest in probiotics, which are defined as live microorganisms that confer a health benefit on the host when administered in adequate amounts [75]. Obviously, probiotic bacteria that are originally isolated from human milk are particularly attractive organisms since they would fulfill

Conclusion

Human milk is a source of bacteria to the infant gut, where they may play a variety of antiinfectious, immunomodulatory, and metabolic roles. In fact, recent studies indicate that the mammary gland contains its own microbiota during late pregnancy and lactation. This bacterial community may differ depending on the individual and the health status of the lactating women. It seems that certain bacteria from the maternal gut can use mononuclear immune cells to colonize, first, the mammary gland

Acknowledgements

This study was partly supported by the FUN-C-FOOD (Consolider-Ingenio 2010) and AGL2010-15420 projects from the Ministerio de Economía y Competitividad (Spain).

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