Although the presence of a commensal microbiota in human milk has been acknowledged since the 1970s, it was not until recently that scientists started elucidating why it is beneficial for both infant and maternal health
No single component is responsible for the known health benefits of human milk. Commensal, mutualistic and potentially probiotic bacteria might all play a part, as do human milk oligosaccharides, which may affect maternal metabolism and are being explored in other contexts beyond the mother-child binomial. For instance, a new randomized controlled trial has shown the role of supplementation with these atypical sugar molecules as a potential way for restoring an altered gut microbiota in patients with irritable bowel syndrome.
However, while scientists are getting a clearer picture of the composition and functional diversity of the bacteria contained in the human milk microbiota, human milk fungi are being neglected.
Meghan Azad’s research group profiled the fungal makeup of breast milk in 271 mothers from the CHILD birth cohort and found detectable fungi—mainly Candida, Alternaria and Rhodotorula—in around 21% of samples. The presence of fungi in the milk was also linked to lower levels of the human milk oligosaccharides disialyllacto-N-tetraose and lacto-N-hexaose. It is likely that those molecules might either inhibit milk fungi or be metabolized by them. The authors, however, acknowledged that other milk components beyond HMOs could also influence milk fungi.
Researchers also saw how the richness of fungi and bacteria in the milk was closely related. That is, in the presence of fungi, the relative abundance of Proteobacteria was lower, while Firmicutes and Bacteroidetes phyla were higher. Those results show that the community of bacteria in breast milk may be shaped by breast milk fungi, with both agonistic and antagonistic interactions between the two kingdoms at the ecological level.
Correlations were also observed between milk fungi, maternal and infant factors (e.g. maternal atopy and early-life antibiotic exposure) and the outdoor environment. Population density in participants’ home cities and season were among the factors that were shown to have the greatest influence on the human milk mycobiota. That might be explained by shifts in outdoor fungi levels, which in turn might affect the presence of milk fungi. For instance, milk samples collected in spring had a lower presence of fungi. In contrast to bacterial milk microbiota composition that was addressed earlier, mode of breastfeeding did not show an effect on mycobiota diversity and composition.
The impact of environmental and perinatal factors on milk fungi is not new.
A previous study led by Mari Carmen Collado from the Spanish National Research Council found that geographical location and mode of delivery were determinants of fungal species in the breast milk of 80 healthy mothers in South Africa, Finland, China and Spain, with a core formed by Malassezia, Davidiella, Sistotrema and Penicillium.
The authors suggest that the overlooked fungi in mother’s milk should be explored further, because they may be important factors in the development of asthma and allergies in children. Yet, their long-term impact on infant and maternal health are unknown at this point. Other unanswered questions in the field include the origins of milk fungi and milk factors and conditions that can influence milk mycobiota.
Jeurink PV, van Bergenhenegouwen J, Jiménez E, et al. Human milk: a source of more life than we imagine. Benef Microbes. 2013; 4(1):17-30. doi: 10.3920/BM2012.0040.
Iribarren C, Törnblom H, Aziz I, et al. Human milk oligosaccharide supplementation in irritable bowel syndrome patients: a parallel, randomized, double-blind, placebo-controlled study. Neurogastroenterol Motil. 2020; e13920. doi: 10.1111/nmo.13920.
Moossavi S, Fehr K, Derakhshani H, et al. Human milk fungi: environmental determinants and inter-kingdom associations with milk bacteria in the CHILD Cohort Study. BMC Microbiology. 2020; 20:146. doi: 10.1186/s12866-020-01829-0.
Arrieta MC, Arévalo A, Stiemsma L, et al. Associations between infant fungal and bacterial dysbiosis and childhood atopic wheeze in a nonindustrialized setting. J Allergy Clin Immunol. 2018; 142(2):424-34. doi: 10.1016/j.jaci.2017.08.041.
Limon JJ, Tang J, Li D, et al. Malassezia is associated with Crohn’s disease and exacerbates colitis in mouse models. Cell Host Microbe. 2019; 25(3):377-88. doi: 10.1016/j.chom.2019.01.007.
Aykut B, Pushalkar S, Chen R, et al. The fungal mycobiome promotes pancreatic oncogenesis via activation of MBL. Nature. 2019; 574(7777):264-7. doi: 10.1038/s41586-019-1608-2.