People with longCOVID, where symptoms persist after recovery from an acute Covid-19 infection, may have distinct differences in the mix of bacteria living in their mouths and lungs, research has shown.

The differences in the lung microbiome are linked to respiratory symptoms, as well as the cognitive and digestive problems known to occur in longCOVID, researchers said.

The findings, published in the European Respiratory Journal, may contribute towards understanding why some people develop long COVID and point to new ways to treat the condition, they said.

"Prior studies have shown some changes in the gut microbiome with long COVID, but Covid is a lung disease and so it makes sense to look to see if the lung microbiome is changed in long COVID," lead author Daniela Alferes de Lima Headley from the University of Dundee, UK, said.

"We know that millions of people worldwide are still affected by long COVID and its cause is still unknown. Long Covid affects multiple organs. The microbiome is important to maintain health throughout the body, so it is a strong candidate for something that may contribute to long COVID," she said.

The study examined 771 people treated for Covid-19 at one of 38 UK hospitals between February 2020 and March 2021.

The participants were divided into four groups based on how bad their symptoms were five months after being discharged from hospital, ranging from 'very severe' with serious physical, mental, and cognitive impairments to 'mild', with few symptoms and good recovery.

Six months after hospital discharge, samples were collected from patients' mouths through saliva and from their lungs through sputum. DNA sequencing was used to identify the microbes present.

By analysing the variety and quantities of microbes and comparing them with patients' symptoms, researchers found that people with very severe long COVID had marked differences in their oral and lung microbiomes compared with those who did not have severe long COVID.

The results also revealed differences linked to specific symptoms, including reduced lung function, in which the lungs cannot hold as much air as normal or air cannot flow in and out as easily.

The researchers also found differences in the mouth and lungs microbiome between people with varying neurocognitive symptoms -- including 'brain fog', difficulties with memory, attention and concentration -- and gut symptoms, including pain, nausea, diarrhoea and loss of appetite.

Among the changes associated with long-COVID symptoms, was an over-abundance of bacteria that produce lactic acid, which was also linked to signs of inflammation found in samples of patients' blood.

Lead researcher James Chalmers, chair of respiratory research at the University of Dundee, UK, said, "People who had the worst symptoms five to six months after COVID infection had big changes in the microbiome in their mouth and lungs."

"We also found that changes in the airway microbiome were linked not just to lung problems, but also to symptoms like brain fog and gut symptoms, suggesting the lung microbiome could have effects on health more broadly," he said.