Measurable variations in the microbial communities inhabiting the mouths and lungs of those living with long Covid — a condition marked by symptoms that continue after the initial infection has cleared — have been identified through scientific investigation. Variations in the bacterial communities inhabiting the lungs appear to be associated with breathing difficulties, along with the neurological and gastrointestinal complications that researchers have documented among people experiencing long Covid.

Published in the European Respiratory Journal, the research may help explain why certain individuals go on to develop long Covid and could open pathways toward new treatment approaches. Daniela Alferes de Lima Headley, the lead author based at the University of Dundee, noted that while earlier work had documented gut microbiome shifts in long Covid patients, the respiratory nature of the disease made the lung microbiome a logical area to investigate, adding that millions of people around the world remain affected by the condition and that its underlying cause has yet to be established.

Research has found that the condition touches a range of bodily systems beyond the lungs. Gut bacteria play a governing role in bodily health across multiple systems, she noted, making the microbiome a plausible factor worth examining in connection with long Covid.

Drawn from 38 National Health Service hospitals across the United Kingdom, the research cohort comprised 771 patients who had received treatment for Covid-19 at some point during the period spanning February 2020 through to March 2021. Four cohorts took shape among the study participants, sorted by the extent of their lingering health difficulties five months after leaving hospital — running from a "very severe" tier marked by physical, mental, and cognitive impairments through to a "mild" tier in which most had largely recovered with few ongoing complaints.

At the half-year mark following their hospital discharge, oral and respiratory specimens — gathered via saliva and sputum respectively — were drawn from the patient group. Microbial composition at the site was determined by running the collected samples through a DNA sequencing process.

When researchers cross-referenced the types and volumes of microbes present against the clinical picture reported by patients, those living with the most debilitating forms of long Covid showed notably distinct bacterial and fungal profiles in both the mouth and lungs relative to people whose long Covid remained less severe. Among the symptom-specific patterns the data uncovered was a drop in lung capacity — a condition where the lungs either struggle to hold their usual volume of air or face resistance when moving air in and out.

Distinctions in the microbial communities of the mouth and lungs also came through in the data, tied to how participants' symptom profiles divided — those reporting neurocognitive effects such as mental fogginess, impaired recall, and trouble sustaining attention or concentration on one side, and those whose complaints centred on the digestive tract, covering abdominal pain, nausea, loose stools, and reduced appetite on the other. Elevated counts of lactic-acid-producing bacteria were among the gut microbiome shifts observed in long-Covid patients, a pattern that also corresponded with inflammatory markers detected in their bloodwork.

Chalmers, who leads respiratory research at the University of Dundee, noted that patients reporting the most persistent symptoms five to six months after a Covid infection showed pronounced shifts in the microbial communities of both the mouth and lungs, and that those airway microbiome changes appeared connected not only to respiratory difficulties but also to conditions such as brain fog and gut-related complaints — a pattern that points toward the lung microbiome playing a broader role in overall health.