The Skin Microbiota

Microbial composition varies across skin regions due to differences in moisture and sebum levels. Oily areas like the forehead support bacteria such as Cutibacterium acnes, while moist regions like the scalp harbor fungi including Malassezia furfur. Dry areas like the forearms contain greater microbial diversity from major bacterial phyla. These environmental conditions determine which microbes can thrive in each location.

Cutibacterium acnes dominates oily skin areas and metabolizes sebum to produce propionic acid, which lowers skin pH and creates a hostile environment for pathogens. This acidification contributes to the skin's innate defense system by restricting pathogen growth. The bacteria's metabolic activity helps maintain skin health and prevent harmful microbial colonization.

Staphylococcus and Corynebacterium species inhabit moist areas such as the armpits and metabolize sweat components to produce antimicrobial compounds. These metabolites inhibit the colonization and growth of harmful microbes, providing a protective function. This antimicrobial activity helps maintain a balanced microbial environment and supports skin health.

Staphylococcus epidermidis colonizes dry areas like the forearms and secretes antimicrobial peptides called bacteriocins. These peptides suppress pathogenic bacteria and help maintain a balanced microbial environment. By producing these antimicrobial compounds, S. epidermidis plays a critical protective role in supporting skin health.

Malassezia furfur, a fungus that colonizes moist and oily regions like the scalp, can trigger dandruff in susceptible individuals. The fungus produces metabolites that provoke host inflammatory responses, disrupting normal skin homeostasis. While generally commensal, overgrowth of M. furfur leads to the characteristic symptoms and scaling associated with dandruff.

Bacteriophages are viruses that infect specific bacterial hosts within the skin microbiome. By targeting and controlling bacterial populations, bacteriophages of the human virome influence bacterial population dynamics and contribute to microbial balance on the skin surface. This viral-bacterial interplay is essential for maintaining dermatological health and resisting infection.

Dry areas like the forearms contain representatives from major bacterial phyla including Actinobacteriota, Bacteroidota, Pseudomonadota, and Bacillota. These regions show greater microbial diversity compared to oily or moist areas. This diverse bacterial community contributes to skin health through various metabolic and antimicrobial functions.