The average temperature of the human body has been steadily declining since the middle of the 19th century, and scientists aren't sure why. A new study suggests one key factor that might play a role in this: gut microbes.

Examining data from patients hospitalized with sepsis – where the body reacts in a dangerously extreme way to infection – as well as from tests on mice, the researchers behind the study looked at the relationship between gut bacteria, changes in temperature, and health outcomes.

That choice of sepsis patients is deliberate because the condition can lead to a variety of temperature fluctuations in the body that are often related to the chances of someone pulling through and recovering.

"We know that temperature response is important in sepsis because it strongly predicts who lives and who dies," says microbiologist and immunologist Robert Dickson from the University of Michigan.

"But we don't know what drives this variation and whether it can be modified to help patients."

The team studied gut bacteria samples taken from 116 people with sepsis, discovering that there were wide variations in the microbiota – and that the variations correlated with changes in the temperature trajectories of the patients.

Bacteria from the Firmicutes phylum were most closely associated with having a higher fever. These bacteria produce important substances for body growth and health and influence the body's immune response and metabolism.

While it's not enough to show that gut bacteria are why our insides have been getting cooler over the last 150 years, it's an interesting hypothesis – and it shows how our gut microbiome is linked to body temperature.

"Arguably, our patients have more variation in their microbiota than they do in their own genetics," says internist Kale Bongers, also from the University of Michigan. "Any two patients are more than 99 percent identical in their own genomes, while they may have literally 0 percent overlap in their gut bacteria."

In further tests on healthy mice with and without a bacteria microbiome, lower base body temperatures were observed in the animals without the bacteria – while treatment with antibiotics also reduced body temperature in the mice.

What's more, across both the humans and the mice, the same family of bacteria seemed to be associated with fluctuations in temperature. The next step is to look at more samples from a broader range of people and to work out what biological mechanisms are underpinning this relationship.

With more research, it's possible that we might be able to develop ways of modifying the gut microbiome specifically to affect body temperature – and that, in turn, could improve the outlook for people with conditions such as sepsis.

"There's a reason that temperature is a vital sign," says Bongers. "It's both easily measured and tells us important information about the body's inflammatory and metabolic state."

The research has been published in the American Journal of Respiratory and Critical Care Medicine.