In seborrhoeic dermatitis, C. acnes may have a positive or at least a neutral role. A study looking at a treatment based on hyaluronic acid noted that as the condition improved the ratio of C. acnes relative to the pathogenic Staphylococcus aureus increased, “re-equilibrating” the skin flora. This is separate to the commensal fungal Malassezia spp implicated as the main causative agent.13,14

Malassezia spp, especially M. restricta and M. globosa, are thought to trigger seborrhoeic dermatitis by over-proliferating and/or causing an over-reaction of the immune system to the yeasts’ metabolites, such as the fatty acids oleic acid, malassezin, and indole-3-carbaldehyde.15

Malassezia spp can also cause or aggravate other skin conditions including pityriasis versicola, Malassezia folliculitis (previously known as Pityrosporum folliculitis), neonatal cephalic pustulosis (a pustular condition on young infants resembling acne), and a pigmented eruption on the chest, back and neck of teenage girls called papillomatosis.16