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Review
. 2024 May 17;12(5):1009.
doi: 10.3390/microorganisms12051009.

Residents or Tourists: Is the Lactating Mammary Gland Colonized by Residential Microbiota?

Ruomei Xu  1 , Grace McLoughlin  2 , Mark Nicol  2 , Donna Geddes  1 , Lisa Stinson  1
Affiliations
Review

Residents or Tourists: Is the Lactating Mammary Gland Colonized by Residential Microbiota?

Ruomei Xu et al. Microorganisms. .

Abstract

The existence of the human milk microbiome has been widely recognized for almost two decades, with many studies examining its composition and relationship to maternal and infant health. However, the richness and viability of the human milk microbiota is surprisingly low. Given that the lactating mammary gland houses a warm and nutrient-rich environment and is in contact with the external environment, it may be expected that the lactating mammary gland would contain a high biomass microbiome. This discrepancy raises the question of whether the bacteria in milk come from true microbial colonization in the mammary gland ("residents") or are merely the result of constant influx from other bacterial sources ("tourists"). By drawing together data from animal, in vitro, and human studies, this review will examine the question of whether the lactating mammary gland is colonized by a residential microbiome.

Keywords: bacteria; colonization; host–microbe interactions; human milk; microbiome.

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Conflict of interest statement

D.G. and L.S. receive a salary from a research grant from Medela AG. This funding body did not have any input into manuscript design, data interpretation, or the decision to publish.

Figures

Figure 1
Figure 1
Human milk contains antimicrobial and anti-adhesive compounds that may prevent bacteria from colonizing the mammary gland. (A): Human milk oligosaccharides (HMOs) can bind bacteria, preventing them from adhering to the mammary epithelium, and increasing the likelihood that they will be flushed during milk ejection. (B): Secretory immunoglobulin A (sIgA) have multiple mechanisms through which they may prevent bacterial adherence to the mammary epithelium. (C): Lysozyme breaks linkages in the peptidoglycan cell wall of Gram-positive bacteria and can disrupt cellular respiration in both Gram positive and negative bacteria if it passes the cell wall. (D): Lactoferrin disrupts the Gram-negative bacterial cell wall via its interaction with lipopolysaccharide and compromises the Gram-positive cell wall via its binding to lipoteichoic acid, allowing other molecules such as lysozyme to penetrate. (E): Lactoferrin sequesters environmental iron required for bacterial growth. (F): Host immune cells present in human milk can produce antibodies against milk bacteria and perform phagocytosis.
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