Gut Bacteria Tied to HIV Reservoir Size and Vaccine Response, Thesis Finds

For people living with HIV, decades of effective antiretroviral therapy (ART, the daily medication that keeps the virus suppressed) have turned the infection into a manageable chronic condition. But researchers keep returning to one unresolved question: what is still happening in the gut? A new doctoral thesis from Karolinska Institutet, built from five linked studies, ties specific gut bacteria to two outcomes patients and clinicians care about: how strongly the body responds to vaccination, and how much dormant virus remains hidden in reservoir cells despite years of effective treatment.

Key takeaways

• A Karolinska Institutet thesis combining five component studies found that specific gut bacteria are associated with both COVID-19 vaccine antibody response and the size of the hidden HIV-1 reservoir in people on long-term antiretroviral therapy (ART).
• People with a stronger antibody and immune-cell response to an mRNA COVID-19 vaccine tended to have lower overall gut bacterial diversity, with Faecalibacterium and Bifidobacterium standing out as markers of that stronger response.
• Faecalibacterium prausnitzii was linked to a smaller HIV-1 reservoir, and Prevotella copri to a larger one, but the thesis did not test whether either bacterium actually causes the difference.
• This article is based on the thesis's published abstract; the full text was not accessible, and the abstract does not state how many participants were in each component study.

Gut Bacteria Linked to HIV-1 Reservoir Size and Vaccine Response

A new doctoral thesis, based on five linked studies in people living with HIV (PLWH), reports that specific species and genera of gut bacteria track with both the size of the HIV-1 reservoir (the pool of dormant, genetically intact virus that survives suppressive treatment) and how strongly a person responds to vaccination. The thesis starts from a baseline observation: compared with HIV-negative controls, PLWH on ART had reduced alpha diversity (a measure of how many bacterial species live in the gut and how evenly they're distributed) and a different overall bacterial makeup (beta diversity), with three species, Streptococcus anginosus, Actinomyces odontolyticus, and Rothia mucilaginosa, significantly more abundant in PLWH. A broader scoping review of the field has reported similar antiretroviral-associated shifts in gut bacterial communities, consistent with that baseline.

Building on that, one component study used shotgun metagenomic sequencing alongside the intact proviral DNA assay (IPDA, a lab test that counts copies of HIV's genetic material still capable of producing virus) in PLWH on long-term ART. People with a smaller intact reservoir had lower species evenness in their gut, and two organisms, Phocaeicola plebeius and Lachnospira sp000437735, were enriched in that low-reservoir group. Looking at individual species, Faecalibacterium prausnitzii, a bacterium often studied for its anti-inflammatory role in the gut, showed a negative association with reservoir size, while Prevotella copri showed a positive one. The same analysis found two metabolic pathways, glycolysis and branched-chain amino acid biosynthesis, running higher in people with larger reservoirs.

A separate pair of studies looked at the gut microbiome's relationship to immune response following SARS-CoV-2 mRNA vaccination (Pfizer-BioNTech) in PLWH and HIV-negative controls, measured 35 days after the shot. Counterintuitively, people with higher antibody titers (IgG levels) and spike-specific CD4+ T-cell responses, two markers of a robust vaccine response, had lower gut bacterial diversity overall. Faecalibacterium and Bifidobacterium were identified as markers of a strong antibody response, while Cloacibacillus tracked with a weaker one; other research has separately connected gut microbiota composition to antibody response after mRNA vaccination. A companion study, which also profiled the virulence-gene content of gut bacteria (the resistome and virulome, the full set of resistance and virulence genes a bacterial community carries), found that low vaccine responders in both PLWH and HIV-negative participants carried more of several bacterial virulence factors, including HlyC and FimG, than high responders did.

Why Vaccine Response and HIV Reservoir Size Matter

For people living with HIV on long-term ART, two of the hardest problems to improve through medication alone are how well the immune system answers a vaccine and how much latent virus remains ready to rebound if treatment stops. A weaker vaccine response means less protection from the vaccines PLWH are routinely advised to get; a larger reservoir is the central obstacle that HIV cure research is trying to shrink. This thesis adds gut bacteria to the list of factors that track with both.

It also found that the choice of ART regimen itself leaves a fingerprint on the gut and mouth. People on integrase strand transfer inhibitors (INSTI, a drug class linked in other research to weight gain) had Faecalibacterium and Bifidobacterium enriched in the gut, compared with Gordonibacter, Megasphaera, and Staphylococcus in people on non-nucleoside reverse transcriptase inhibitors (NNRTI). In the mouth, Veillonella was more abundant in the INSTI group. Separately, in people with a body mass index (BMI) over 25, Bifidobacterium and Dorea were enriched in the gut and Veillonella in the mouth, a pattern that echoes the INSTI findings and may be relevant given INSTI's known association with weight gain, though the thesis does not draw that causal line itself.

How the Five Studies Were Designed

The thesis combines five separately conducted studies, using 16S rRNA gene sequencing (a technique that identifies bacteria by one shared stretch of their DNA) and shotgun metagenomic sequencing (which reads nearly all the DNA in a sample, bacterial genes included) on fecal and, in one study, oral swab samples from PLWH compared with HIV-negative controls. The reservoir study added the IPDA, a dedicated assay built specifically to count intact copies of the HIV-1 provirus. The published abstract for this thesis does not state how many participants were enrolled in each of the five studies, which makes it hard to judge how much statistical weight these associations can carry on their own.

What This Means If You're Living With HIV

If you're living with HIV and keeping up with ART, none of this is actionable yet. These are observational associations, not evidence that changing your gut bacteria, through diet, probiotics, or anything else, would shrink your reservoir or boost your vaccine response; the thesis didn't test any intervention, only measured who already had which bacteria. It's reasonable to mention gut health to your HIV care team if you have ongoing digestive symptoms, but your personal microbiome makeup isn't a validated tool for predicting how you'll respond to a vaccine or a cure strategy; these patterns were observed across groups, not confirmed as an individual-level predictor.

Why the Findings Don't Prove Cause and Effect

Every result above is a correlation, not a tested cause. None of the five component studies manipulated the microbiome or tracked it over time to see which came first, the bacterial shift or the outcome, so it's equally possible that something else, immune status, diet, or the HIV infection itself, drives both the bacterial pattern and the reservoir or vaccine outcome. This article is based on the thesis's published abstract, hosted on Karolinska Institutet's open archive; the full text of the thesis was not available for review, so nothing beyond what the abstract states is included here.

What's Next for This Research

Four of the five studies behind this thesis have already been published individually in peer-reviewed journals, including Frontiers in Microbiology, NPJ Biofilms and Microbiomes, Scientific Reports, and Gut Pathogens, with a fifth manuscript still in preparation. Follow-up work that tracks these bacterial signatures over time, or tests whether shifting them changes vaccine response or reservoir size, would be needed to move from association to anything clinically useful.

Common Questions About Gut Bacteria and HIV Treatment

What is the HIV-1 reservoir, and why do researchers want to measure it? The reservoir is the pool of HIV-infected cells that stay dormant but genetically intact despite years of ART; it's what allows the virus to rebound within weeks if someone stops treatment, which is why a smaller reservoir is one of the clearest markers HIV cure research is chasing.

Does this mean changing my gut bacteria will improve my vaccine response? Not based on this research. The thesis found statistical associations between certain bacteria and vaccine response, not proof that changing your microbiome would raise your antibody levels, and no intervention was tested.

Why did people with lower gut bacterial diversity have a stronger vaccine response? The thesis doesn't explain why, and its design can't establish a causal chain; it only reports that, in this group, lower alpha diversity accompanied a stronger measured antibody and T-cell response to the mRNA COVID-19 vaccine.

Do these findings apply to everyone, or just people with HIV? The thesis studied people living with HIV, often alongside HIV-negative controls for comparison, so it doesn't establish whether the same bacteria-to-outcome patterns hold in people without HIV.

References

1. Aswathy Narayanan (2026). Impact of gut microbiome and inflammation on clinical aspects of HIV infection. https://doi.org/10.69622/31913892
2. Zsófia Gáspár, Blin Nagavci, Bálint Gergely Szabó, Botond Lakatos (2024). Gut Microbiome Alteration in HIV/AIDS and the Role of Antiretroviral Therapy—A Scoping Review. Microorganisms. https://doi.org/10.3390/microorganisms12112221
3. The role of intestinal microbiota in the humoral response to SARS-CoV-2 after mRNA-1273 vaccination. NIH. https://pubmed.ncbi.nlm.nih.gov/40634526/
4. Christian Gaebler, Shane D. Falcinelli, Elina Stoffel, et al. (2021). Sequence Evaluation and Comparative Analysis of Novel Assays for Intact Proviral HIV-1 DNA. Journal of Virology. https://doi.org/10.1128/jvi.01986-20