Competition between different bacteria species in the gut is what keeps humans healthy, a study has suggested.
The research, published in the journal Science, uses mathematical theory to suggest how microbes in the gut maintain stability.
Scientists previously thought bacteria were cooperating, but the Oxford University study suggests competition between them makes for good health.
Prof Kevin Foster said bacteria acted like "trees competing in a jungle".
"The assumption has always been that because these bacteria are doing us good, the communities must be cooperating with one another.
"What our work suggests, based on a wide-ranging mathematical analysis, is that competition may be key to a healthy gut," he said.
The communities of microbes carried inside humans - known as the microbiome - help with:
- the breakdown of food
- protection from harmful pathogens
- keeping a healthy immune system
The research took three years to complete, and its conclusions have been corroborated in a separate study of bacteria in mice.
The study suggested if bacteria were cooperating, it could actually "destabilise" the body's systems and lead to poor health.
Prof Foster, who is one the authors of the research and works in the department of evolutionary biology at Oxford University, added humans may have evolved to "act as ecosystem engineers that manipulate general, system-wide properties of their microbial communities to their benefit".