A new study has shown that people with chronic fatigue syndrome have abnormal levels of specific gut bacteria - providing even more evidence that the condition isn't "just in a person's head".

For decades, millions of people have reported experiencing symptoms now associated with a condition called chronic fatigue syndrome - a debilitating disease that causes brain fog, severe pain, and exhaustion so extreme, patients can't go about their daily lives, and sometimes can't even get out of bed. But a physical cause has been elusive, leaving many feeling that their condition isn't being taken seriously.

It was only in 2015 that the US Institute of Medicine detailed a comprehensive way to diagnose chronic fatigue syndrome/myalgic encephalomyelitis (ME/CFS), and earlier this year, scientists linked the condition to faulty cell receptors in immune cells for the first time - which explains why the side effects can be so varied and hard to pin down.

But there are still no effective treatments for the disease, and no cure - some commonly prescribed treatments for the condition have been cognitive behavioural therapy and exercise, neither of which have any evidence to support they work, and could actually be doing more harm than good.

Now, new research has shown that patients with ME/CFS have abnormal levels of specific gut bacteria - and those levels change depending on the severity and type of symptoms they have.

"Individuals with ME/CFS have a distinct mix of gut bacteria and related metabolic disturbances that may influence the severity of their disease," said one of the researchers, Dorottya Nagy-Szakal from Columbia University's Mailman School of Public Health.

"By identifying the specific bacteria involved, we are one step closer to more accurate diagnosis and targeted therapies," added lead researcher Ian Lipkin.

The study adds to research from last year, which showed that up to 80 percent of patients with ME/CFS could be accurately diagnosed by looking at their gut bacteria.

And it's also known that up to 90 percent of ME/CFS patients have irritable bowel syndrome (IBS), so the latest research began to untangle the specific gut bacteria changes associated with each condition.

The team followed 50 ME/CFS patients and 50 healthy controls, who had been carefully matched. They tested the number of bacterial species in faecal samples, and looked at the immune molecules in their blood.

They found that seven distinct intestinal bacterial species were strongly associated with ME/CFS, so much so that an elevated presence of all of them could predict a diagnosis.

The strains were:

Faecalibacterium

Roseburia

Dorea

Coprococcus

Clostridium

Ruminococcus

Coprobacillus

There were also specific changes seen in the gut bacteria of those who had chronic fatigue syndrome with IBS, and those who didn't have IBS.

Interestingly, when the team measured bacterial metabolic pathways - the ways that bacteria break down food and send signals to the brain - there were clear differences between the healthy controls and the ME/CFS group.

There were also measurable differences depending on the severity of a patient's symptoms, which suggests that are different subtypes of ME/CFS that could be identified.

While this study involved only a small sample size, with further verification, this could be the first step towards coming up with targeted ways to not only diagnose the debilitating disease, but also treat it.

"Our analysis suggests that we may be able to subtype patients with ME/CFS by analysing their fecal microbiome," said one of the team, Brent L. Williams.

"Subtyping may provide clues to understanding differences in manifestations of disease."

The research has been published in the journal Microbiome.