"The immunized mice responded with a more proactive behavioral coping response to stress, a strategy that has been associated with stress resilience in animals and humans," said Christopher Lowry, PhD, associate professor at CU-Boulder and the senior author of the new research.
‘An injection of M. vaccae activates the individual's immunoregulatory responses to protect from inappropriate inflammation’
The researchers found that mice given a heat-killed preparation of M. vaccae
responded with approximately half as many flight and avoidance behaviors when challenged by an aggressor, as compared to untreated mice, during the first hour of the experiment.
The immunized mice continued to show decreased levels of submissive behaviors one to two weeks after treatment. M. vaccae
treatment reduced stress-induced colitis, as measured by cellular damage to the colon and infiltration of immune cells, by approximately 50 percent, as compared to untreated mice.
The research underscores the importance of an organism's microbiome in preventing and coping with inflammation-related diseases and psychiatric conditions. Microbiomes are made up of teeming communities of microorganisms, including environmental bacteria.
A microbiome's diversity is increasingly recognized to play key roles in immunoregulation - controlling autoimmune and allergic responses, decreasing vulnerability to infection, and other regulatory health functions. Previous research has suggested that stress-related conditions such as post-traumatic stress disorder may be due to a failure of immunoregulation.
"An injection of M. vaccae
is not designed to target a particular antigen the way a vaccine would, but instead activates the individual's immunoregulatory responses to protect from inappropriate inflammation," said Lowry.
The "hygiene hypothesis," also known as the "Old Friends" hypothesis, posits that modern sanitary measures, antibiotics and dietary changes have greatly reduced human exposure to environmental bacteria and other immunoregulatory organisms, thereby reducing the diversity of human microbiomes and their ability to suppress inappropriate inflammation. A lack of exposure to these organisms is thought to contribute to the current epidemic of inflammatory disease in modern urban societies.
"Humans and human ancestors used to encounter these microorganisms in abundance every day, but modern life doesn't facilitate those interactions," said co-author Rob Knight, PhD, director of the Center for Microbiome Innovation at UC San Diego.
"We are continuing to look at how these naturally-occurring bacteria can modulate the immune system in ways that might be beneficial," said Lowry.