To the confusion of medical researchers, being teetotal is no guarantee you'll avoid the kind of liver disease that tends to affect heavy drinkers. Now, one strange medical case could get us closer to understanding why that happens.

The patient, a 27-year-old man in China, was suffering from a form of liver inflammation called nonalcoholic steatohepatitis, and had a rather strange history of becoming drunk without actually… drinking.

To get a high blood alcohol reading of somewhere around 400 milligrams per decilitre, most of us would need to down at least a dozen shots of hard liquor.

Not this guy. The subject of this case study only needed a good supply of fruit juice and a few plates of carbs.

As much as it sounds like an inventive excuse to give to traffic police, it is actually possible for gut microflora to take carbohydrates in our food and ferment them into an intoxicating level of ethanol.

It's a rare condition known as auto-brewery syndrome (ABS), and this patient's case could help to explain the liver damage that happens as intestinal bacteria turn last night's pasta into their very own homebrew.

It's not exactly common, and from the handful of cases studied in detail, the syndrome appears to be caused by an overrepresentation of the brewer's favourite fungal friend, Saccharomyces cerevisiae.

This time, however, the researchers needed to look a little further for a suitable microbe to blame for the non-drinker's states of intoxication and resulting liver disease.

"We initially thought it was because of the yeast, but the test result for this patient was negative," says paediatrician Jing Yuan from Capital Institute of Paediatrics in Beijing.

"Anti-yeast medicine also didn't work, so we suspected [his disease] might be caused by something else."

A dig through the patient's faeces managed to identify a potential candidate in the form of a microbe called Klebsiella pneumoniae.

While some strains will take the opportunity to rise up as a nightmarish, disease-causing superbug, it's usually a boring old gut bacterium that most of us nurture in our digestive tract. But in the gut of this patient, K. pneumoniae was anything but boring.

The researchers compared his microbes with those found in 43 other people with non-alcoholic fatty liver disease (NAFLD), and a further 48 healthy individuals.

Over the course of the study, they found the abundance of K. pneumoniae increased by around 0.02 percent in most individuals. In their subject, the change was closer to a whopping 19 percent.

With a culprit in sight, the team successfully isolated strains of the bacterium that weren't just tolerant of alcohol, but could generate significant amounts of it with oxygen both present and absent.

Tellingly, the abundance of these two fermenting strains as well as their ability to churn out alcohol were higher in those with NAFLD than in healthy volunteers. In fact, 60 percent of the NAFLD patients had strains of the bacterium that could generate medium to high amounts of alcohol, compared with just 6 percent of those in the healthy group.

To help verify a link between the K. pneumoniae and fatty liver disease, the researchers infected the guts of mice with the bacterium. Sure enough, two months later, their livers were already showing serious signs of scarring.

None of this means K. pneumoniae is the primary cause of all cases of liver disease that don't have clear links to alcohol consumption. But it could be a good starting place for those concerned.

"NAFLD is a heterogeneous disease and may have many causes," says Yuan.

"Our study shows K. pneumoniae is very likely to be one of them. These bacteria damage your liver just like alcohol, except you don't have a choice."

Exactly how and why this bacterial strain sets up a microbrewery in some hosts and not others isn't clear. But the team plans on further investigating the microbe's role in liver disease.

"Having these bacteria in your gut means your body is exposed to alcohol constantly," says fellow researcher Di Liu from the Chinese Academy of Sciences.

"So does being a carrier mean you would have higher alcohol tolerance? I'm genuinely curious!"

As for the patient whose fermenting bowels inspired this discovery, we can only hope his days of being accused of being a closet alcoholic are over, along with any hangovers he doesn't deserve.

This research was published in Cell Metabolism.