People with very severe long Covid have a different mix of bacteria in their mouths and lungs, a new UK study found. The study was published in the European Respiratory Journal, CIDRAP and The Microbiologist reported.

The research is part of PHOSP-COVID, a large UK study of people who were in the hospital with Covid. It included 771 adults treated between February 2020 and March 2021. Six months after they left the hospital, the team took saliva samples from their mouths and sputum samples from their lungs. They used DNA sequencing to see which microbes were there.

In the group with very severe symptoms, the bacteria in the mouth were much less diverse than in the group with mild symptoms, CIDRAP reported. That group had the most breathlessness, fatigue and weak lung function. The Microbiologist reported that differences in the lung microbiome were linked to breathing symptoms, as well as thinking and digestive problems.

"We know that millions of people worldwide are still affected by long Covid and its cause is still unknown," said lead author Daniela Alferes de Lima Headley of the University of Dundee, The Microbiologist reported. The researchers said the findings may help explain why some people get long Covid and could point to new treatments.

The study was observational, so it cannot show that the bacteria cause long Covid symptoms.

The team split people into four groups, from mild to very severe, based on their symptoms. The work was funded by the UK's National Institute for Health and Care Research and UK Research and Innovation.

What we know: A UK study of 771 adults linked very severe long Covid to different bacteria in the mouth and lungs.
What we don't: Whether the bacteria cause symptoms, and whether treating them would help.