In a new study, researchers found that the variety and volume of bacteria in the gut, known as the microbiome, may influence the severity of COVID-19 as well as the magnitude of the immune system response to the infection.
Imbalances in the make-up of the microbiome may also be implicated in persisting inflammatory symptoms, dubbed ‘long COVID’.
The research was conducted by a team at The Chinese University of Hong Kong and elsewhere.
COVID-19 is primarily a respiratory illness, but the evidence suggests that the gut may also have a role.
The gut is the largest immunological organ in the body and its resident microbes are known to influence immune responses.
In the study, the team wanted to find out if the gut microbiome might also affect the immune system response to COVID-19 infection.
They obtained blood and stool samples and medical records from 100 hospital inpatients with laboratory-confirmed COVID-19 infection between and from 78 people without COVID-19.
The severity of COVID-19 was classified as mild in the absence of X-ray evidence of pneumonia; moderate if pneumonia with fever and respiratory tract symptoms were detected; severe if patients found it very difficult to breathe normally; and critical if they needed mechanical ventilation or experienced organ failure requiring intensive care.
The team showed that the make-up of the gut microbiome strongly differed between patients with and without COVID-19, irrespective of whether they had been treated with drugs, including antibiotics.
COVID patients had higher numbers of Ruminococcus gnavus, Ruminococcus torques and Bacteroides dorei species than people without the infection.
And they had far fewer of the species that can influence immune system response, such as Bifidobacterium adolescentis, Faecalibacterium prausnitzii and Eubacterium rectale.
Lower numbers of F. prausnitzii and Bifidobacterium bifidum were particularly linked to infection severity.
And the numbers of these bacteria remained low in the samples collected up to 30 days after infected patients had cleared the virus from their bodies.
COVID-19 infection prompts the immune system to produce inflammatory cytokines in response.
In some cases, this response can be excessive (‘cytokine storm’), causing widespread tissue damage, septic shock, and multiorgan failure.
Analysis of the blood samples showed that the microbial imbalance found in the COVID patients was also linked to raised levels of inflammatory cytokines and blood markers of tissue damage, such as C-reactive protein and certain enzymes.
This suggests that the gut microbiome might influence the immune system response to COVID-19 infection and potentially affect disease severity and outcome, say the researchers.
One author of the study is Yun Kit Yeoh.
The study is published in Gut.
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