The human gut microbiota is involved in immune regulation, metabolism, and skin homeostasis. In recent years, gut microbiota alterations have been linked with several inflammatory skin disorders, such as atopic dermatitis (AD), psoriasis, and hidradenitis suppurativa (HS). This systematic review synthesises current evidence on gut microbiota composition and functional alterations in these dermatoses. A comprehensive literature search was conducted in the PubMed database, identifying studies from inception to January 2025. Eligible studies included human observational, interventional, and genetic studies investigating gut microbiota alterations in AD, psoriasis, or HS, using microbiome profiling or genetic causal-inference approaches. Studies lacking control groups or relying on culture-based techniques were excluded. Sixty-two studies were included: 38 on AD, 22 on psoriasis and 5 on HS, with three addressing more than one disease. In AD, most studies focused on paediatric populations, leaving a knowledge gap regarding adult-specific data. Reduced alpha-diversity and decreased abundance of Faecalibacterium prausnitzii, Bifidobacterium spp., and Akkermansia muciniphila were recurrent findings. In psoriasis, in addition to dysbiosis, microbial metabolic pathways were also found to be altered. In HS, data remain limited, but increased Ruminococcus gnavus and reduced alpha-diversity have been reported, mirroring findings in inflammatory bowel diseases. Gut microbiota has been increasingly implicated in skin inflammation. Despite advances in microbiota analysis, significant gaps remain-especially in adult AD and HS. Future research should prioritize standardised methodologies, larger and more diverse cohorts, and leverage emerging tools such as Mendelian randomization and AI-based models to develop precision medicine interventions.
