Microbiome. Conceptual image of the microbiome of human skin. Coloured scanning electron micrograph (SEM) of bacteria superimposed on an image of a hand. There are around 1000 species of bacteria from 19 phyla found on human skin. Skin flora is usually non-pathogenic, and either commensal (not harmful to their host) or mutualistic (offer a benefit). One benefit bacteria can offer is preventing transient pathogenic organisms from colonizing the skin surface, either by competing for nutrients, secreting chemicals against them, or stimulating the skin's immune system. However, resident microbes can cause skin diseases and enter the blood system, creating life-threatening diseases, particularly in immunosuppressed people.