It is now well established that our health depends on the interaction between gut bacteria and the immune system. It has been suggested that the rich diversity of bacteria in our gut is the result of the two systems co-evolving and working on each other. Now, a new study provides evidence for one side of the theory – that the immune system influences the evolution of gut bacteria.

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The study shows evidence that a healthy immune system helps shape the evolution of gut bacteria.

The study, from the Instituto Gulbenkian de Ciência (IGC) in Portugal, is published in the journal Nature Communications.

From experiments with lab mice, the IGC researchers discovered that when the immune system is defective, the mix of gut bacteria changes, as does the pace and the manner in which they adapt – however, it is not easy to predict what these ways are.

The team suggests their findings support the idea that treatments for immune-related intestinal disorders – such as inflammatory bowel disease (IBD) – should not be based on generalized but on personalized medicine, because this takes into account the unique composition of the individual’s gut bacteria.

The gut is a complex, ever-changing environment that gut bacteria are highly responsive to. They need to adapt and evolve quickly, for example, just to cope with changes in the food we eat every day.

Because the job of the immune system is to keep a look-out for potential disease-causing agents, it must monitor closely what is happening in the gut to make sure it does not treat friendly microbes as enemies. But how does it do this?

The new study does not exactly explain how it happens, but it provides sufficient evidence to show that it does, using a common gut bacterium as a model. It shows that a healthy immune system helps shape the destiny of gut bacteria in predictable ways.

For the study, the team investigated Escherichia coli in mice. E. coli is an abundant gut microbe and one of the first to colonize human and mouse intestines after birth.

The researchers compared the guts of healthy mice with those of mice lacking white blood cells – a key component of the immune system.

They found that the digestion and metabolism of the healthy mice adapted quickly to changes in diet, but these changes took longer in the immune-deficient mice.

When they investigated further, the researchers noticed that across the mice with healthy immune systems, the genetic changes in gut bacteria that occurred as the diet varied were broadly the same.

However, they found the genetic changes in gut bacteria were more divergent and mixed among the immune-deficient mice, to the point where it was very difficult to predict what course the evolution of their gut bacteria would take.

First author João Barroso-Batista, a PhD student at IGC, says:

We observed that this feature is due to changes in the composition of the community of bacteria in the intestine, which is more similar across individuals with a healthy immune system, and is quite diverse in animals with an immune compromised system.”

The authors conclude that their findings show while it is possible to predict how gut bacteria will evolve in healthy individuals, the same cannot be said for those with faulty immune systems.

Therefore, it would seem that treatment of individuals with immune-related intestinal diseases such as IBD need to take into account the unique composition of their gut bacteria – personalized as opposed to generalized medicine.

Meanwhile, Medical News Today recently learned of another study that showed good health relies on some less abundant gut microbes. Reporting in the journal Cell Host & Microbe, researchers from the University of Oregon in Eugene explain how they discovered some species of gut bacteria do not need a large presence to have a big influence.