The skin microflora, or microbiota, and its genomic diversity, or microbiome, are significant factors in inflammatory skin diseases, whether as a cause or through secondary infection.

The commensal flora lives in a healthy balance in the microbiota with different species modifying the levels of each other to promote homeostasis. However, they can become opportunistic pathogens if conditions change. Similarly, microbes new to the flora can be pathogenic by unbalancing the homeostasis or by triggering an epidermal T-cell immune response.3,11

For example, acne is associated with certain genera of bacteria. The most important of these is Cutibacterium, and particularly C. acnes, previously known as Propionibacterium acnes. These respond to the changing skin conditions in puberty as sebaceous glands increase sebum production due to increasing androgens. Corynebacterium species such as Corynebacterium simulans also proliferate.3

The mix of microbes can also affect severity. In rosacea, while the causes are not properly understood, one factor linked to triggering the immune response and subsequent inflammation is increased levels of Demodex mites and associated Bacillus bacteria. However, severity has been linked to the relative levels of certain bacteria, a positive association with Gordonia and negative with Geobacillus.12