NEW DELHI, October 12, 2026 : Long COVID may be associated with significant changes in the communities of bacteria living in the mouth and lungs, potentially offering new clues to why some people continue to experience health problems months after recovering from an acute COVID-19 infection, according to a study published in the European Respiratory Journal.
Researchers found that people experiencing more severe long COVID symptoms had distinct differences in their oral and lung microbiomes compared with those who had recovered more successfully or experienced fewer persistent symptoms.
The findings also revealed associations between changes in the lung microbiome and a broad range of health problems, including breathing difficulties, cognitive impairment, commonly described as “brain fog”, and digestive symptoms.
Scientists said the findings could help improve understanding of long COVID and potentially support the development of new approaches to diagnosis and treatment. However, further research will be needed to establish whether these microbial changes contribute to the condition or occur as a consequence of it.
Study Examines 771 COVID-19 Patients Across the UK
The research examined 771 people who had received treatment for COVID-19 at 38 hospitals across the United Kingdom between February 2020 and March 2021.
Participants were classified into four groups according to the severity of their symptoms five months after hospital discharge.
The categories ranged from very severe long COVID, involving substantial physical, mental and cognitive impairment, to mild symptoms and relatively good recovery.
Six months after discharge, researchers collected saliva samples to study the oral microbiome and sputum samples to examine microorganisms present in the respiratory tract.
Using DNA sequencing, the team identified the microbes present in the samples and analysed their variety and relative abundance. These results were then compared with participants’ symptoms and levels of impairment.
The analysis revealed pronounced differences in the oral and lung microbiomes of people with very severe long COVID compared with those who had fewer persistent symptoms.
Lung Microbiome Changes Linked to Breathing Difficulties
One of the key findings was an association between changes in the lung microbiome and respiratory symptoms.
These included reduced lung function, a condition in which the lungs may hold less air than expected or airflow may become restricted.
The researchers observed that the composition of airway microorganisms differed according to the severity of patients’ ongoing health problems.
The findings suggest that studying the respiratory microbiome could provide additional insights into the biological processes associated with persistent symptoms following COVID-19.
However, the study does not establish that changes in bacteria directly cause lung damage or prolonged respiratory illness.
Possible Connection With Brain Fog and Digestive Symptoms
The research also identified differences in the mouth and lung microbiomes among people experiencing varying degrees of neurocognitive and gastrointestinal symptoms.
Neurocognitive symptoms included:
- Brain fog and difficulty thinking clearly.
- Problems with memory and concentration.
- Difficulties maintaining attention.
Digestive symptoms included abdominal pain, nausea, diarrhoea and loss of appetite.
The associations are particularly noteworthy because long COVID can affect multiple organ systems, extending beyond the respiratory symptoms commonly associated with the initial infection.
Researchers suggested that changes in the airway microbiome might be connected with broader health processes. Nevertheless, the precise biological mechanisms behind these associations remain uncertain.
Bacteria Producing Lactic Acid Among the Findings
Among the microbial changes identified in the study was an increased abundance of certain bacteria that produce lactic acid.
These changes were also associated with indicators of inflammation detected in blood samples from patients.
Inflammation is one of several biological processes being investigated in efforts to understand long COVID. The findings raise the possibility that interactions between microorganisms and the body’s immune response could be relevant to persistent symptoms.
However, the researchers have not established whether these bacteria contribute to inflammation, whether inflammation alters the microbiome, or whether other factors influence both.
Further studies will be necessary to determine the significance of these findings.
Researchers Highlight the Importance of the Lung Microbiome
Lead author Daniela Alferes de Lima Headley of the University of Dundee in the UK said earlier research had identified changes in the gut microbiome among people with long COVID.
She explained that examining the lung microbiome was also important because COVID-19 primarily affects the respiratory system and the microbiome plays a role in maintaining health throughout the body.
James Chalmers, chair of respiratory research at the University of Dundee and the study’s lead researcher, said people experiencing the most severe symptoms five to six months after infection showed substantial changes in the microbiomes of their mouths and lungs.
He also highlighted the association between airway microbiome changes and symptoms extending beyond the lungs, including brain fog and gastrointestinal problems.
The researchers believe these findings could broaden scientific understanding of the complex and varied effects of long COVID.
What the Findings Mean for Long COVID Treatment
Long COVID remains a significant health concern because some people continue to experience symptoms for months or longer after their initial infection.
The condition can affect physical functioning, mental well-being, concentration, breathing and everyday activities. Its underlying causes are still being investigated, and different biological mechanisms may contribute to symptoms in different individuals.
The new findings suggest that the oral and lung microbiomes could become useful areas of investigation in future research.
Scientists may be able to examine whether particular microbial patterns can help identify people at risk of persistent symptoms or reveal potential targets for treatment.
However, the study does not demonstrate that altering the microbiome, taking probiotics or using antibiotics can treat or prevent long COVID. Such approaches would require separate clinical evidence.
Conclusion: The study provides new evidence of an association between long COVID severity and changes in the bacteria found in the mouth and lungs. The links with respiratory problems, brain fog and digestive symptoms offer promising avenues for further investigation, but more research is needed before these findings can be translated into clinical treatments.
