The World Health Organization says an estimated 300 million people suffer from depression, and there are known links between a patient's physical and mental health.

Scientists in Belgium now believe that a wide range of gut bacteria can produce chemicals that significantly impact the brain, including several microorganisms linked -- positively or negatively -- to mental health.

The experiment, known as the Flemish Gut Flora Project, examined depression data and stool samples from more than 1,000 people and found that two types of bacteria were "consistently depleted" in those who suffered from depression. This held true even if patients were on anti-depressants.

Scientists' understanding of how the gut and brain are linked is in its early stages, and the researchers acknowledged that their findings, published in the journal Nature Microbiology, could be considered controversial.

"The notion that microbial metabolites can interact with our brain -- and thus behaviour and feelings -- is intriguing," said lead researcher Jeroen Raes, from the department of Microbiology and Immunology at KU Leuven University.

"Until now, most of the studies were or in mice or in small-scale human studies, with mixed and contradictory results," he told AFP.

The team repeated the study on 1,063 people from the Netherlands and a third group of clinically depressed patients in Belgium, and got similar results.

Raes stressed, however, that while the experiment showed a clear link between the levels of certain bacteria in the gut and an individual's mental well-being, that didn't mean that one thing directly caused the other.

The two microbe groups, coprococcus and dialister, are know to have anti-inflammatory properties.

"We also know that neuro-inflammation is important in depression. So, our hypothesis is that somehow these two are linked," said Raes.

- 'Bugs as drugs' -

Depression -- a treatable but debilitating condition that affects how an individual behaves and feels -- is sometimes referred to as a "silent epidemic", and is a major driver of the some 800,000 suicides that occur each year worldwide.

Anti-depressants are now the most common prescribed drug in many countries and Raes said his team's research could pave the way for new, smarter treatments for the illness.

"I really think there is a future in this: using cocktails of human-derived bacteria as treatment –- bugs as drugs, as they say," he said.

The team behind the research studied the genomes of more than 500 types of gut bacteria and analysed their ability to produce a set of neuroactive compounds -- chemicals shown to affect brain function.

They found several that could produce compounds linked to a variety of mental processes.

Raes said recent technological and medical advances had allowed researchers to zero in on the gut when looking into possible causes of mental health issues.

"The field went for the obvious diseases first -– obesity, diabetes, colon cancer, irritable bowl syndrome," he said.

"But then evidence started coming in from animal studies," he added. "The last few years, the gut-brain axis field really exploded. It's really, really exciting at the moment."