The human microbiome project was a major undertaking by the National Institutes of Health, with a fairly simple mission: understand the bacterial communities living in and on the human body, and the potential impact these communities may have on health. Hundreds of individuals donated everything from feces to nasal secretions. However, one key system was ignored – human milk. That’s right – the microbiome of human milk was not studied.
Probably some of this had to do with a long standing myth that human milk was sterile. Why study something without bacteria, right? But, as we have quickly learned – human milk is far from sterile. The average baby consuming 800 mL/27 ounces of human milk will received between 100,000 and 10,000,000 million bacteria from human milk per day.
Fortunately, research into the human milk microbiome has continued despite this oversight by the Human Microbiome Project. It appears that nine “operational taxonomic units” (generally closely related species based on DNA analysis of the bacteria) are extremely common in most mothers studied to date: Streptococcus, Corynebacteria, Bradyrhizobiaceae, Staphylococcus, Serratia, Ralstonia, Propionibacterium, Pseudomonas, and Sphingomonas. These nine groups typically account for more than 50% of total bacteria. Bififobacterium and Lactobacillus are also common, but less universal (Fernandez et al., 2013).
The microbiota of milk appears to be quite stable (Fernandez et al., 2013), although a few factors appear to shape the composition. First, mothers with higher BMIs (in the obese range) produce colostrum with more Lactobacillus, and mature milk with more Staphylococcus and less Bifidobacterium (Cabera-Rubio et al., 2012). Cabera-Rubio and colleagues (2012) also found that greater pregnancy weight gain predicted more Staphylococcus in the milk in a small study of 18 mothers, half obese and half of normal weight.
But here is the really neat part – guess what else altered the milk microbiota? Type of delivery. Mothers who had caesarian sections had a different milk microbiota than mothers who had a vaginal delivery. And the variation continued – mothers undergoing emergency caesarians after laboring had milk microbiotas closer to those of women who delivered vaginally than women with elective caesarians.
Read full, original article: Human milk has a microbiome – and the bacteria are protecting mothers and infants!