Mice carrying a set of friendly microbes that are usually found in humans fail to develop a proper immune system and are left susceptible to illness as a result. The findings in the journal Cell, a Cell Press publication, show that animals have coevolved with and rely on their own very special array of microbial partners. As far as our immune systems are concerned, not just any bug collection will do.

"Human microbe-colonized mice have gut immune systems that look essentially identical to germ-free mice," said Dennis Kasper of Harvard Medical School. "Even though they have the same number and diversity of bacteria, their immune systems don't develop properly."

The findings come at a time when the importance of the microbiome (meaning the totality of microbes in an environment, such as our gut) for health and well-being is becoming increasingly clear. The results might have implications for understanding the health consequences of our shifting diets, our excessive use of antibiotics, and our modern-day obsession with showers and antibacterial household cleansers, the researchers say.

"Because the intestinal microbiota can regulate immune responses outside the gut, the absence of the 'right' gut microbes may conceivably shift the balance toward disease in individuals genetically predisposed to autoimmune diseases," they write, noting that our relationship with our gut microbiome today may be threatened by a combination of heavily processed foods, frequent treatment with antibiotics, and advances in hygiene.

Kasper's group compared germ-free mice to mice colonized by either a proper mouse microbiome or a human one. The mice were otherwise living in carefully controlled, germ-free "bubbles." The number and diversity of those mouse and human microbial collections were about the same; it was their species makeup that differed.

Mice with their normal complement of microbes were also better than human microbe-colonized mice at fighting off a Salmonella infection. The researchers conclude that "a host-specific microbiota appears to be critical for a healthy immune system."

Although modern medicine and technology may offer alternative ways to fight disease, Kasper says, "the current prevalence of autoimmune diseases - such as asthma, multiple sclerosis, and inflammatory bowel disease - may be, at least in part, the consequence of the increasing vulnerability of the coevolved human-microbe relationship."