From the Friend to the Foe—Enterococcus faecalis Diverse Impact on the Human Immune System Original paper

What was reviewed?

This review examined the role of Enterococcus faecalis as both a commensal microbiome organism and an opportunistic pathogen, with emphasis on its interactions with the immune system, microbiome balance, dysbiosis, and systemic disease. The authors evaluated mechanistic evidence describing how E. faecalis regulates immune tolerance, maintains intestinal homeostasis, modulates inflammatory signaling, and transitions into a pathogenic organism when microbiome balance becomes disrupted. The review focused on microbial immune signaling pathways, probiotic immunomodulatory effects, dysbiosis-driven overgrowth, virulence gene acquisition, immune evasion strategies, and their role in chronic inflammatory and infectious diseases.

Who was reviewed?

The review evaluated data from human microbiome studies, probiotic clinical trials, mechanistic cell studies, and animal models examining immune signaling, inflammatory regulation, infection progression, and microbiome interactions. These studies assessed commensal and pathogenic E. faecalis strains in the gastrointestinal tract, immune tissues, and systemic infection models to characterize its microbiome-associated immune effects and disease mechanisms.

What were the most important findings?

Enterococcus faecalis colonizes the human gastrointestinal tract early in life and persists as a stable microbiome member, typically representing about 1% of adult gut microbiota at concentrations of 10⁵–10⁷ CFU per gram of stool. It supports microbiome homeostasis by metabolizing nutrients, producing vitamins, regulating intestinal pH, and suppressing pathogen growth through bacteriocin production. It actively regulates immune tolerance by stimulating TLR2 signaling and inducing anti-inflammatory cytokines such as IL-10 and TGF-β, which strengthen epithelial barrier integrity and prevent excessive inflammation.

Probiotic strains increase IgA production and promote Th1 immune responses, thereby enhancing antimicrobial defense and reducing inflammatory disease relapse. However, dysbiosis disrupts bile acid regulation and removes microbial growth controls, which allows E. faecalis expansion and increased virulence gene expression. Pathogenic strains express virulence factors such as aggregation substance, enterococcal surface protein, and biofilm-associated adhesins, which enhance immune evasion, suppress macrophage inflammatory signaling, inhibit autophagy, and resist neutrophil killing. These mechanisms allow bacterial persistence, intestinal barrier translocation, and systemic infection.

What are the greatest implications of this review?

This review demonstrates that Enterococcus faecalis functions as a microbiome-dependent immune regulator whose effects depend on microbiome balance and host conditions. In microbiome stability, it promotes immune tolerance, supports gut barrier function, and contributes to immune homeostasis. In dysbiosis, it expands pathologically, evades immune clearance, promotes inflammatory signaling, and contributes to infections such as endocarditis, urinary tract infections, and intra-abdominal infections. These findings establish E. faecalis as a key pathobiont whose expansion reflects microbiome instability and increased inflammatory disease risk. Its abundance, virulence gene expression, and immune signaling effects represent important microbiome signatures for assessing immune dysfunction, dysbiosis progression, and infection susceptibility.