What a probiotic actually does in the gut
A probiotic is a live micro-organism that, taken in adequate amounts, confers a health benefit. That is the formal definition, and the two conditions inside it, live and adequate amounts, are where the whole subject actually lives.
Here is what these organisms are doing in a gut, in plain English.
Your child already has about a hundred trillion of them
The human gut carries a resident population of bacteria that outnumbers the body's own cells. It establishes in the first years of life, shaped by birth, feeding and diet, and by school age it looks broadly like an adult's.
These are not passengers. The gut microbiome performs work the human body cannot do for itself.
Four things they do
They finish digesting food. Some carbohydrates, the ones called fibres and oligosaccharides, pass through the small intestine undigested because humans lack the enzymes to break them down. Gut bacteria have those enzymes. What they do with that material is ferment it, and the products of that fermentation are short-chain fatty acids, which the cells lining the colon use as their preferred fuel.
They occupy the space. A well-populated gut wall leaves fewer places for anything else to attach. This is called competitive exclusion and it is one of the oldest observations in the field.
They talk to the immune system. The majority of the body's immune tissue sits along the gut, which makes sense: it is the largest surface where the outside world meets the inside. The resident bacteria are in constant conversation with it.
They make things. Gut bacteria synthesise vitamin K and several B vitamins. They also produce neurotransmitters, which is the part that has drawn most of the recent attention.
The gut-brain part, stated carefully
The gut and the brain are physically connected by the vagus nerve, and they exchange signals constantly in both directions. That is settled anatomy, not a theory.
The gut also holds much of the body's serotonin, made by specialised cells in the gut lining. Work in mice has shown that particular gut bacteria promote that production. (Yano et al., Cell, 2015.)
And in the laboratory, when 91 human gut bacteria were screened for their ability to produce GABA, the brain's main calming messenger, a strain of Lactobacillus brevis converted up to 90 per cent of its raw material. (Barrett et al., 2012.)
Those are real findings about real capabilities. What they establish is a mechanism, and mechanism is where every useful field starts.
What to take from it
Gut bacteria finish digesting food, occupy space that would otherwise be available to something else, work alongside the immune system, and manufacture vitamins and neurotransmitters. The gut and brain are wired together and signal in both directions.