New Study Links Long COVID to Oral Microbiome Changes

A new long COVID oral microbiome study found distinct bacterial patterns in the mouths and airways of patients with the most severe ongoing symptoms. Researchers analyzed samples from 771 adults previously hospitalized with COVID-19 across 38 UK hospitals. The findings link those patterns with lung function, cognitive and gastrointestinal symptoms, and blood markers of inflammation.

TL;DR

• Researchers sequenced oral and sputum samples from 771 adults about six months after COVID-19 hospital discharge.

• Participants in the very severe long COVID cluster showed the most distinct oral and airway microbial profiles.

• Oral profiles tracked with lung function and neurocognitive symptoms, and sputum profiles tracked with gastrointestinal symptoms.

• The observational design cannot show whether these bacterial shifts caused, followed, or simply accompanied long COVID.

New Study Finds Oral Microbiome Differences in Severe Long COVID

The study appeared in the European Respiratory Journal on October 12, 2026. It was led by the University of Dundee and forms part of PHOSP-COVID, the UK's national post-hospitalization COVID-19 study. People with very severe long COVID had clearly different microbial communities in both the mouth and the lower airway.

The two sample types told different parts of the story. Oral microbial profiles were associated with impaired lung function and persistent neurocognitive symptoms. Sputum profiles were associated with gastrointestinal symptoms, including pain, nausea, diarrhea, and loss of appetite. The European Respiratory Society's announcement reports one further finding. An overabundance of lactic acid-producing bacteria was linked to worse outcomes and to signs of inflammation in blood.

These are group-level differences. An individual patient with long COVID will not necessarily carry any particular microbial profile. The milder clusters also showed fewer differences.

How Did Researchers Study Oral Bacteria in 771 Patients?

All 771 participants had been hospitalized for COVID-19 between February 2020 and March 2021 at one of 38 UK hospitals. Hospital care in that period commonly involved oxygen therapy, corticosteroids, and antibiotics, and each can reshape oral and airway bacteria. Hospital oral care protocols may also include antiseptic rinses such as chlorhexidine, which alter oral bacterial communities. These exposures matter when interpreting a microbiome snapshot taken months later.

How Researchers Collected Oral and Airway Samples

Samples were collected about six months after discharge. The oral sample was saliva, which carries bacteria shed from the tongue, cheeks, plaque, and oropharynx. Sputum was coughed up from the lower airway. Sputum passes through the mouth on its way out, so some oral bacteria mix into it, and the two compartments are not fully independent.

The team used 16S ribosomal RNA gene sequencing. This method reads a gene shared by nearly all bacteria and sorts organisms by its variable regions. It usually identifies bacteria reliably to the genus level and reports their relative abundance. It cannot show whether bacteria were alive, which genes they expressed, or how many were present in absolute terms.

How Researchers Classified Long COVID Severity

Participants were grouped by health status about five months after discharge, using severity clusters previously defined within PHOSP-COVID:

• Very severe, with serious physical, mental, and cognitive impairment

• Severe, with serious physical and mental impairment without cognitive problems

• Moderate, with mainly cognitive difficulties

• Mild, with few symptoms and good recovery

Severity was assessed about a month before microbial sampling. Every participant had been hospitalized. People who developed long COVID after a mild infection, a large share of all cases, were outside the study population.

Which Oral Bacteria Were Associated With Long COVID Symptoms?

The headline bacterial finding concerns lactic acid-producing bacteria. These organisms ferment sugars into lactic acid. The group includes Streptococcus and Lactobacillus species, both core members of a healthy mouth. Streptococcus is typically the most abundant genus in saliva, and several lactobacilli are sold as probiotics.

That makes the finding harder to interpret than the headline suggests. An overabundance of this group does not imply that a pathogen invaded. It may reflect a shift in community balance, changes in diet or saliva, or reduced competition from other bacteria.

Dentists know these organisms from caries ecology, where acid-tolerant streptococci and lactobacilli thrive in low-pH plaque. This study did not examine caries, plaque, or periodontal status. Its findings therefore cannot be extended to dental disease risk.

The researchers also linked these bacterial patterns to inflammatory markers in blood. That is a correlation between two measurements taken around the same period. It does not show which came first.

How Were Microbiome Changes Linked to Brain Fog and Lung Function?

The symptom associations differed by sample type. The table summarizes the reported findings.

A link between oral bacteria and brain fog has an obvious clinical alternative explanation. Cognitive impairment and fatigue make daily brushing and interdental cleaning harder. Patients with severe symptoms may also eat differently, take more medications, and have drier mouths. Any of these could shift oral bacteria. The microbiome difference may follow the illness and its consequences without driving it, and the study design cannot separate these paths.

Could Oral Bacteria Contribute to Long COVID?

The researchers suggest that the mouth, airway, gut, and brain may interact in long COVID. Two biological routes are plausible. Oral bacteria are regularly microaspirated into the lower airway, so oral communities can seed the lungs. Microbial products can also stimulate immune responses that keep inflammation active. Both remain hypotheses, and neither was tested in this study.

Earlier work adds context. A 2021 JCI Insight study took oral swabs from hospitalized patients during acute infection. Higher levels of lipopolysaccharide-producing Prevotella and Veillonella were associated with symptoms lasting beyond 10 weeks. Sampling before long COVID developed is informative about timing, and that cohort numbered only 27 patients. Three questions remain open:

• Did the microbial differences exist before SARS-CoV-2 infection?

• Did the infection, hospital treatment, or recovery period change these communities?

• Could the changes contribute to ongoing symptoms?

A single sample at six months cannot answer any of them.

What Do the Findings Mean for Dentists?

The study does not create a dental screening test or treatment protocol for long COVID. Its practical value lies in how dentists assess and support these patients.

Long COVID belongs in the medical history, along with current medications. Antidepressants, antihistamines, and other drugs used for symptom management can reduce salivary flow. Patients reporting dry mouth may benefit from established measures such as saliva substitutes for xerostomia, along with stronger fluoride prevention when caries risk rises.

Fatigue and cognitive symptoms can erode home care. Shorter appointments, simplified hygiene instructions, and power toothbrushes may help patients keep up. Periodontal findings should be charted and managed on their own merits, using standard periodontitis staging and grading. These steps protect oral health. No evidence shows that periodontal therapy or plaque control improves long COVID symptoms.

Oral findings that may reflect a wider problem warrant contact with the patient's physician. Examples include unexplained dry mouth and swallowing difficulty. Recording oral findings, medications, and referrals in a dental treatment plan keeps care coordinated across visits. Readers interested in other microbial layers of the mouth can explore the research on the oral virome.

Could Saliva Testing Help Diagnose Long COVID?

Saliva suits microbiome research well. It is noninvasive, easy to collect repeatedly, and carries bacteria from across the mouth and throat. A convenient sample is still a long way from a diagnostic test.

This study identified group-level patterns associated with severity. A clinical test would need several further layers of evidence:

• Independent validation, in cohorts that were not used to discover the pattern

• Diagnostic accuracy, with sensitivity and specificity measured against clinical diagnosis

• Reproducibility, across laboratories, sampling methods, and sequencing platforms

• Clinical comparison, showing added value over established symptom-based assessment

The wider evidence is also inconsistent. A 2026 pilot study in Enfermedades Infecciosas y Microbiología Clínica compared people with persistent COVID with matched healthy controls. It found no difference in oral microbiota diversity, and salivary cytokines were also similar. No saliva microbiome test has been validated to diagnose long COVID, and this study does not provide one.

What Are the Study's Limitations?

Several features of the design limit how far the results can be taken:

• Observational design, which identifies associations without establishing cause

• No preinfection samples, so each participant's baseline microbiome is unknown

• A single time point, which leaves recovery or persistence of these profiles unknown

• A hospitalized population, infected in 2020 and early 2021, before widespread vaccination and later variants

• Confounding exposures, including antibiotics, corticosteroids, oxygen therapy, comorbidities, smoking, and oral hygiene

• 16S sequencing limits, which report relative abundance without measuring bacterial activity

In the ERS announcement, Professor James Chalmers notes that the team does not know whether the changes preceded infection. He also notes that it remains unknown whether the microbiome recovers in later years.

Bottom Line

The PHOSP-COVID analysis found that severe long COVID after hospitalization is associated with distinct oral and airway microbial profiles. Oral profiles tracked with lung function and cognitive symptoms, sputum profiles tracked with gastrointestinal symptoms, and lactic acid-producing bacteria correlated with inflammation. The findings do not establish cause, a diagnostic saliva test, or a treatment. For dentists, they support careful history-taking, dry-mouth management, and practical home-care support for affected patients.