It turns out that people experiencing a major depression have differences in their gut microbiome (community of microbes) when compared to healthy people who are not depressed. A persistent and prolonged period of extreme sadness or depression is called a major depressive disorder (MDD).
A team of researchers (in both China and the US) analyzed stool samples from 311 people with either MDD or healthy and not-depressed (the control group). They used modern genetic sequencing to see what microbes were in the stools. They found differences in 47 bacterial species, 3 bacteriophages (a virus that infects bacteria), and 50 fecal metabolites - which suggested to the researchers that depression is characterized by gut microbiome problems (it's imbalanced or out of whack).
There actually was a "signature composition" of gut microbes in the depressed persons, all of whom were unmedicated. They found higher levels ("increased abundance") of 18 bacterial species in people with MDD (mainly belonging to the genus Bacteroides) and 29 were less common (mainly belonging to Eubacterium and Blautia), when compared to healthy persons.
The researchers point out that other studies also find the gut microbiome to be imbalanced in MDD, and there are animal experiments showing that the gut microbiome has a role in causing MDD (e.g. transplanting gut microbes from a depressed person into a rat results in the rat exhibiting depressive behaviors).
Excerpts from The Scientist: Distinct Microbiome and Metabolites Linked with Depression
The human gut microbiome is a world in miniature, populated by a chatty community of bacteria, viruses, fungi, and protozoa nestled within various gastrointestinal niches. Over the past decade, researchers have linked disturbances within this complicated microbial society to a variety of diseases. Major depressive disorder (MDD) is one such condition, but the studies have been small and the findings imprecise. ...continue reading "Gut Microbiome Is Altered In Persons With Major Depression"