Virome

Overview

The virome is the collection of all viruses inhabiting a given ecosystem — in the gut, this means primarily bacteriophages (phages), which constitute ~90% of the intestinal virome. While microbiome research has historically focused on bacteria, mounting evidence shows the virome is not a passive bystander but an active ecological force: phages shape bacterial community composition through selective predation, horizontal gene transfer, and modulation of bacterial fitness. In several conditions, virome-based classifiers outperform bacterial signatures for disease prediction.

The gut virome is the "dark matter" of the microbiome — poorly characterized relative to the bacteriome, but increasingly recognized as a driver of dysbiosis rather than merely a consequence.

Composition

Bacteriophages (~90%)

• Caudovirales (tailed phages): Siphoviridae, Myoviridae, Podoviridae — the dominant order in the healthy gut
• CrAss-like phages: The most abundant and stable phages in the human gut, infecting Bacteroides species
• Temperate phages: Integrated as prophages in bacterial genomes; can be induced by stress (antibiotics, oxidative stress, metal exposure)

Eukaryotic Viruses (~10%)

• Plant-derived viruses (dietary origin)
• Human viruses (enteroviruses, adenoviruses — typically low abundance in healthy individuals)
• Endogenous retroviruses (integrated in the human genome)

Virome in Disease

Colorectal Cancer

The CRC fecal virome shows increased network connectivity — phage-bacteria interaction networks become more complex and interconnected. Virome dysbiosis persists even after surgical resection, suggesting the virome changes are not simply a consequence of tumor presence but may reflect a stable ecological state ^[1].

Schizophrenia

124 viral operational taxonomic units (vOTUs) are enriched in schizophrenia (primarily Siphoviridae and Flandersviridae). A virome-based classifier achieved AUC 93.2% — outperforming both bacterial and mycobiome models for disease discrimination ^[2], ^[3].

Parkinson's Disease

The Tetz group has published a series of studies linking gut phages to PD pathogenesis. Key findings:

• Bacteriophages targeting lactococcus (lytic phages) are enriched in PD patients ^[4].
• Phage-mediated killing of commensal bacteria may precede and precipitate the bacterial dysbiosis observed in PD ^[5].
• Combined bacteriophage and bacterial toxin exposure in the gut may contribute to neurodegeneration through the gut brain axis ^[6].
• Brain virome dysbiosis detected in PD and MSA patients ^[7].

Necrotizing Enterocolitis

A critical finding: virome convergence occurs ~10 days before NEC onset — phage community composition shifts dramatically before clinical disease appears. Phage-mediated killing of commensal bacteria may precipitate the proteobacteria bloom that characterizes NEC ^[8]. This positions the virome as a potential upstream trigger, not downstream consequence, of dysbiosis.

Autism Spectrum Disorder

Gut phageome alterations detected in ASD, with disease-specific viral community structures distinguishing ASD children from healthy controls ^[9], ^[10].

PCOS

Phage-Lactobacillus coevolution supports vaginal eubiosis in healthy women; disruption of this phage-bacteria balance is observed in PCOS ^[11].

Cancer Immunotherapy Response

The gut virome predicts immunotherapy response with AUC 0.768 vs 0.664 for bacteria-only models. Responder-enriched phages target SCFA producers (faecalibacterium prausnitzii, roseburia); non-responder phages target clostridium/bacteroides fragilis ^[12].

Long COVID

Reduced phage diversity in long COVID limits natural pathobiont predation, potentially contributing to persistent proteobacteria enrichment and bacterial translocation ^[13].

Phage Therapy

Phage therapy — using lytic bacteriophages to selectively kill pathogenic bacteria — is experiencing a resurgence as antibiotic resistance escalates:

• Safety: Systematic review confirms favorable safety profile with few serious adverse events ^[14].
• Specificity: Phages are highly specific to their bacterial hosts, theoretically sparing the commensal community (unlike broad-spectrum antibiotics).
• Cardiometabolic applications: Phage therapy explored for targeting proteobacteria pathobionts in metabolic syndrome ^[15].
• Pancreatic cancer: Phage-based peptide delivery systems explored for targeting intratumoral bacteria.

Virome-Bacteriome-Metabolite Interactions

The virome does not operate in isolation. In schizophrenia, tripartite analysis revealed:

• Phage abundance correlates with bacterial host abundance (predator-prey dynamics)
• Phage-mediated bacterial lysis releases metabolites that influence host neurotransmitter pathways
• Virome-bacteriome-metabolite interaction networks are fundamentally reorganized in disease ^[3]

Open Questions

• Metal effects on phages: Do heavy metals directly affect phage stability or host range? Prophage induction under metal stress could reshape the virome.
• Phage-metal resistance transfer: Phages are major vectors for horizontal gene transfer — do they spread metal resistance genes alongside ARGs?
• Temporal dynamics: The NEC virome convergence finding suggests phage shifts precede bacterial dysbiosis. Is this pattern general across diseases?
• Therapeutic targeting: Can phage cocktails be designed to selectively remove metal-tolerant pathobionts while sparing Fe-S-dependent commensals?

Cross-References

• gut microbiome — The virome as the overlooked component
• proteobacteria — Phylum whose bloom may be triggered by phage-mediated commensal killing
• dysbiosis — Virome dysbiosis as upstream trigger
• antimicrobial resistance — Phage therapy as alternative to antibiotics
• gut brain axis — Phage-toxin-neurodegeneration pathway
• probiotics — Phage-probiotic coevolution in health
• firmicutes — SCFA producers targeted by responder vs. non-responder phages