Breast milk is inherently sterile (unless the mother has a severe bacterial infection). It usually gets contaminated with the skin microbiota during feeding.

So no, I wouldn't count on L. reuteri being found in a random sample of breast milk.

Breast milk is not sterile.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2643565/

That's hotly debated. Even the study you cite cannot rule out skin contamination.

But unequivocally showing the existence of entero-mammary pathway (translocation of maternal gut bacteria from the gut to the mammary glands) would be very exciting news indeed.

In the context of this conversation, does it matter whether the cause is skin contamination or not?

Yes, because if you are buying breast milk from a woman who used a breast pump, then the skin contamination is very very little compared to actual breast suckling.

A seller:

https://cosprings.craigslist.org/hab/d/colorado-springs-brea...

So you're telling me I should actually be drinking all of this e-girl bath water I bought?

The article says that L. reuteri is present in other places in the body besides the gut.

You can get HIV from breast milk, it's not sterile.

HIV is an infectious virus. Also, bacteria in milk is usually a sign of mastitis. I'm explicitly talking about healthy mothers.

> Breast milk is inherently sterile

There is nothing sterile in the human body. Not even the brain.

You've put more confidence in your statement than the scientific consensus allows.

A brain microbiome is hotly debated, no concrete proof (yet?). I personally also have a hard time believing in a healthy bacterial blood microbiota, but it's also proposed.