Researchers at Oregon State University (OSU) have found the bacterial communities that live inside human bodies are quite similar and stable when times are good, but when stress enters the equation, those communities can react very differently from person to person.

Published this week in Nature Microbiology and referred to as a microbiological version of the "Anna Karenina principle," the new paradigm gets its name from the opening line of the novel "Anna Karenina" by the famed 19th century Russian writer Leo Tolstoy: "All happy families are alike; each unhappy family is unhappy in its own way."

"When microbiologists have looked at how microbiomes change when their hosts are stressed from any number of factors – temperature, smoking, diabetes, for example – they've tended to assume directional and predictive changes in the community," explained Rebecca Vega Thurber, an assistant professor at OSU and corresponding author on the study.

However, Thurber noted in a news release, "after tracking many datasets of our own we never seemed to find this pattern but rather a distinct one where microbiomes actually change in a stochastic, or random, way."

By surveying the literature on microbial changes caused by perturbation, lead author Jesse Zaneveld of the University of Washington-Bothell, Thurber and her student Ryan McMinds at OSU found those stochastic changes to be a common occurrence.

The findings, Thurber said, indicate that "when microbiomes are happy they are all similar in their composition but during stress or unhappiness they change in a multitude of distinct ways... We think this is an important emerging paradigm for thinking about microbiome data."

Similarly, in humans, when healthy our microbiomes look alike, "but when stressed each one of us has our own microbial snowflake," she said. "You or I could be put under the same stress, and our microbiomes will respond in different ways – that's a very important facet to consider for managing approaches to personalized medicine. Stressors like antibiotics or diabetes can cause different people's microbiomes to react in very different ways."

Humans and animals are filled with symbiotic communities of microorganisms that often fill key roles in normal physiological function and influence susceptibility to disease.

Studies of microbiome dynamics have typically looked for patterns that shift microbiomes from a healthy stable state to a dysbiotic stable state; dysbiosis refers to the microbial communities being out of their natural balance, which can result in the interruption of basic biological functions for the host person or animal.

The researchers believe their new paradigm has key implications for a more personalized approach to antibiotic therapy, management of chronic diseases and other aspects of medical care.