In an effort to pinpoint potential triggers leading to inflammatory responses that eventually contribute to depression, researchers are taking a close look at the immune system of people living in today's cleaner modern society.

Rates of depression in younger people have steadily grown to outnumber rates of depression in the older populations and researchers think it may be because of a loss of healthy bacteria.

In an article published in the December issue of Archives of General Psychiatry, Emory neuroscientist Charles Raison, MD, and colleagues say there is mounting evidence that disruptions in ancient relationships with microorganisms in soil, food and the gut may contribute to the increasing rates of depression.

According to the authors, the modern world has become so clean, we are deprived of the bacteria our immune systems came to rely on over long ages to keep inflammation at bay.

"We have known for a long time that people with depression, even those who are not sick, have higher levels of inflammation," explains Raison.

"Since ancient times benign microorganisms, some times referred to as 'old friends,' have taught the immune system how to tolerate other harmless microorganisms, and in the process, reduce inflammatory responses that have been linked to the development of most modern illnesses, from cancer to depression."

Experiments are currently being conducted to test the efficacy of treatments that use properties of these "old friends" to improve emotional tolerance. "If the exposure to administration of the 'old friends' improves depression," the authors conclude, "the important question of whether we should encourage measured re-exposure to benign environmental microorganisms will not be far behind."

Raison is associate professor in the Department of Psychiatry and Behavioral Sciences at Emory University School of Medicine. Co-authors include Christopher A. Lowry, PhD, Department of Integrative Physiology and Center for Neuroscience, University of Colorado, and Graham A. W. Rook, BA, MB, BChir, MD, FSB, Department of Infection, Windeyer Institute for Medical Sciences, University College London.

The study was funded by the National Institutes of Health, the National Science Foundation, the Georgia Department of Human Services and the Centers for Disease Control and Prevention, as well as the Bill and Melinda Gates Foundation's Grand Challenge Explorations initiative, and the GALTRAIN Marie Curie Early Stages Training Network, funded by the European Union.

Arch Gen Psychiatry, Inflammation, Sanitation, and Consternation: Loss of Contact With Coevolved, Tolerogenic Microorganisms and the Pathophysiology and Treatment of Major Depression, 2010;67(12):1211-1224

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