The interaction between the microbiota and the immune system may also play a role in other diseases in adults, including those caused at least in part by chronic inflammation from hyperactive immune systems.
“Gut bacteria have figured out a way to network with our immune system so it doesn’t attack them,” said Sarkis K. Mazmanian of the California Institute of Technology.
A look at microbes in the human body.
The microbiota apparently sends signals that dampen the “inflammatory response,” a crucial defense also thought to play a role in a variety of diseases, including many forms of cancer, the “metabolic syndrome” caused by obesity, diabetes and heart disease.
The theory is that one reason some people may be prone to these diseases is that they are missing certain microbes. One anti-inflammatory compound produced by a bacterium appears to cure the equivalent of colitis and multiple sclerosis in mice, both of which are caused by misfiring immune systems, Mazmanian found.
Role in obesity?
Similarly, studies indicate that gut dwellers secrete messengers to cells lining the digestive tract to modulate key hormones, such as leptin and ghrelin, which are players in regulating metabolism, hunger and a sense of fullness.
Pregnant women often take antibiotics, and young children can get several rounds to fight ear and other infections, which can kill off these companions. Farmers commonly add antibiotics to animal feed to fatten their animals faster.
“We may have a generation of children growing up without the proper bacteria to regulate their leptin and ghrelin,” Blaser said.
Obese people appear to have a distinctive mix of digestive bacteria that make them prone to weight gain. Thin mice get fatter when their microbiota is replaced with the microbes of obese animals.
“Our ancient microbiome is losing the equilibrium it used to have with the host — us — and that has profound physiological consequences,” said Blaser, who published his concerns in a paper in the journal Nature in August.
Microbes and the mind
Clues also are emerging about how microbes may affect the brain. Manipulating gut microbiomes of mice influences their anxiety and activity, Swedish researchers reported in January in the Proceedings of the National Academy of Sciences.
“This may have implications for new lines of thinking to address some of the psychiatric problems you see among humans,” said Sven Pettersson, a professor of host-microbial interaction at the Karolinska Institute. “Together with genetic susceptibility, this may influence what doctors classify as autism or ADHD.”
In another experiment involving mice, a Canadian-Irish team reported in August that bacteria in the gut appear to influence brain chemistry, and corresponding behaviors such as anxiety, stress and depression, via the vagus nerve.
“What we’ve shown is, you change behavior as well as make changes in the brain,” said John Bienenstock, director of the Brain-Body Institute at McMaster University. “Now we have direct proof how that happens. That’s why this is exciting.”
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