How the Gut Microbiome Influences Immune Response to Infection: Insights from Clinical Research

Introduction

The gut microbiome plays a central role in regulating host immunity. Beyond digestion, it actively shapes immune responses through interactions with epithelial barriers, immune cells, and metabolic signalling pathways. A growing body of research has demonstrated that the gut microbiome contributes to immune homeostasis, inflammatory regulation, and susceptibility to infection.¹, ²

One key mechanism involves microbial metabolites such as short-chain fatty acids, particularly butyrate, which support intestinal barrier integrity and exert anti-inflammatory effects.⁴ In addition, commensal bacteria influence cytokine production and immune cell activation, thereby modulating both innate and adaptive immune responses.², ⁴ Disruptions to this ecosystem, commonly referred to as gut dysbiosis, have been linked to increased vulnerability to infections and impaired immune responses.², ³

Importantly, real-world data suggest that gut microbiome imbalance is highly prevalent in modern populations. Findings from a large-scale study analysing stool samples from 1,000 individuals reveal that approximately 83% of the population may exhibit varying degrees of gut dysbiosis. Even more concerning is that nearly 40% show moderate to severe dysbiosis—a significant level of imbalance comparable to that observed in COVID-19 patients. This widespread disruption, characterised by reduced levels of beneficial bacteria, may be associated with suboptimal immune function.⁷

Taken together, these observations highlight that gut microbiome imbalance is not a niche phenomenon, but a widespread issue with potential implications for immune resilience. To better understand how this relationship manifests during infection, clinical studies have begun to examine microbiome alterations in the context of specific diseases, including COVID-19.

The Gut Microbiome and Infection: Evidence from COVID-19

A clinical study conducted by researchers at The Chinese University of Hong Kong investigated changes in the gut microbiome of patients with COVID-19 during hospitalisation. The study analysed faecal samples from 15 patients and compared them with those of healthy individuals and patients with community-acquired pneumonia.⁶

Alterations in Microbiome Composition

The study found that patients with COVID-19 exhibited significant alterations in their gut microbiome, characterised by:

• Enrichment of opportunistic pathogens, including Clostridium hathewayi, Actinomyces viscosus, and Bacteroides nordii
  

• Depletion of beneficial commensal bacteria, such as Faecalibacterium prausnitzii, Eubacterium rectale, and Roseburia⁶

These beneficial bacteria are known to support immune regulation and anti-inflammatory processes. Their depletion suggests a shift towards a pro-inflammatory and less resilient immune environment.

Notably, these microbiome alterations were observed even in patients who had not received antibiotics, indicating that the changes were associated with the infection itself rather than treatment effects.⁶

Association with Disease Severity

Further analysis revealed that specific bacterial species were correlated with COVID-19 severity:

• Higher levels of Coprobacillus, Clostridium ramosum, and C. hathewayi were associated with more severe disease

• Lower levels of Faecalibacterium prausnitzii, an anti-inflammatory bacterium, were associated with milder disease⁶

These findings suggest that the baseline composition of the gut microbiome may influence how the body responds to infection.

Interaction with ACE2 and Viral Load

The study also identified relationships between gut bacteria and SARS-CoV-2 viral activity.

Certain Bacteroides species, including Bacteroides dorei and Bacteroides ovatus, were found to be inversely associated with viral load. These bacteria are known to downregulate the expression of ACE2, a receptor used by SARS-CoV-2 to enter host cells.⁶

This suggests a potential mechanism by which the gut microbiome may influence susceptibility to infection and viral replication.

Persistence of Dysbiosis After Recovery

One of the most striking findings was that gut microbiome imbalance persisted even after clearance of the virus and resolution of respiratory symptoms.⁶

This indicates that infection may have a prolonged impact on the gut microbiome, potentially affecting recovery and long-term immune function.

Discussion and Insights

The findings from this clinical study reinforce the concept that the gut microbiome is not merely a passive bystander during infection, but an active participant in shaping disease outcomes.

Several key insights emerge:

1. Immune modulation
   The gut microbiome influences immune signalling pathways, including cytokine responses, which are critical in determining the severity of infection.⁶
   

2. Barrier function and inflammation
   Loss of beneficial bacteria may weaken intestinal barrier integrity and promote systemic inflammation, creating conditions that favour disease progression.⁶
   

3. Microbiome as a risk modifier
   Baseline microbiome composition may partially explain differences in individual responses to infection, including susceptibility and severity.⁶
   

4. Long-term implications
   Persistent dysbiosis following infection suggests that recovery is not limited to viral clearance, but may involve restoration of microbial balance.⁶

These findings align with broader research demonstrating that respiratory viral infections can inherently disrupt the gut microbiome, thereby increasing the risk of secondary infections.³, ⁵ From a clinical management perspective, this highlights the importance of judicious antibiotic use during viral pneumonitis to avoid exacerbating baseline gut dysbiosis. Looking ahead, strategies to support microbiome balance, including dietary modifications (e.g., fibre-rich and probiotic-containing foods) to enhance butyrate production, may be explored as potential approaches to support overall health and immune resilience during infection and recovery.

References

1. Hashimoto T, Perlot T, Rehman A, et al. ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation. Nature. 2012;487:477–481.
   

2. Ma WT, Pang M, Fan QL, et al. The commensal microbiota and viral infection: a comprehensive review. Front Immunol. 2019;10:1551.
   

3. Hanada S, Pirzadeh M, Carver KY, et al. Respiratory viral infection induced microbiome alterations and secondary bacterial pneumonia. Front Immunol. 2018;9:2640.
   

4. Mendes V, Galvao I, Vieira AT. Mechanisms by which the gut microbiota influences cytokine production. J Interferon Cytokine Res. 2019;39:393–409.
   

5. Yildiz S, Mazel Sanchez B, Kandasamy M, et al. Influenza A virus infection impacts microbiota dynamics. Microbiome. 2018;6:9.
   

6. Zuo T, Zhang F, Lui GCY, et al. Alterations in gut microbiota of patients with COVID-19 during hospitalisation. Gastroenterology. 2020;159:944–955.
   

7. The Chinese University of Hong Kong. 40% of Hong Kong people show gut dysbiosis comparable to COVID-19 patients. Available from: https://www.med.cuhk.edu.hk/press-releases/40-of-hong-kong-people-show-gut-dysbiosis-comparable-to-that-of-covid-19-patients-cuhk-microbiome-immunity-formula-hastens-recovery-of-covid-19-patients-and-offers-hope-to-boost-immunity