The Skin Microbiota

The human skin serves as a complex ecosystem inhabited by a diverse community of microorganisms, including bacteria, fungi, and viruses. This microbiome plays a critical role in maintaining skin health and defending against pathogenic invaders. The composition of microbial communities varies significantly across different regions of the body, influenced primarily by the local levels of moisture and sebum.

Regional Variation in Skin Microbiota
Cutibacterium acnes predominantly colonizes sebaceous or oily areas such as the forehead. These bacteria metabolize sebum, producing propionic acid, which lowers the pH of the skin. This acidification creates a hostile environment for many pathogenic organisms, contributing to the skin's innate defense system. Similarly, moist and oily regions such as the scalp support the growth of fungi, particularly Malassezia furfur. While generally commensal, an overgrowth of M. furfur is implicated in conditions like dandruff due to its ability to disrupt normal skin homeostasis.

Microbial Activity in Moist and Dry Sites
Moist environments, such as the armpits, foster communities of Staphylococcus and Corynebacterium species. These bacteria produce metabolites, some of which have antimicrobial properties that inhibit the colonization and growth of harmful microbes. On the other hand, dry areas like the forearms show greater microbial diversity and include representatives from major bacterial phyla such as Actinobacteriota, Bacteroidota, Pseudomonadota, and Bacillota. In these regions, Staphylococcus epidermidis plays a protective role by secreting antimicrobial peptides, including bacteriocins, which suppress pathogenic bacteria and support a balanced microbial environment.

Role of Viruses in Skin Homeostasis
In addition to bacterial and fungal residents, the skin microbiome includes viruses, notably bacteriophages. These viruses infect specific bacterial hosts, thereby influencing bacterial population dynamics and contributing to microbial balance on the skin surface. This intricate interplay among microbial taxa underscores the importance of the skin microbiome in maintaining dermatological health and resisting infection.