Clinical improvement and scalp microbiome restructuring following a 28-day piroctone olamine shampoo intervention

To evaluate the clinical efficacy of a piroctone olamine-containing shampoo in individuals with oily dandruff and to characterize associated temporal changes in bacterial and fungal scalp microbiota.

An open-label, single-arm, longitudinal study was conducted in 41 volunteers with oily dandruff over 28 days. Participants applied the test shampoo three times weekly. Clinical assessments (erythema, pruritus, desquamation index and sebum levels) were performed at baseline, Day 14, and Day 28. Scalp microbiome samples were collected by standardized swabbing and analyzed using full-length 16S rRNA gene and ITS amplicon sequencing. Microbial community dynamics were evaluated using compositional data analysis, including centred log-ratio transformation, robust Aitchison distance, compositional tensor factorization and linear mixed-effects modelling.

Significant clinical improvement was observed by Day 28, with marked reductions in erythema and pruritus. Sebum levels increased during the study period (p < 0.01), indicating that symptom improvement occurred independently of reduced sebum production. Bacterial community analysis revealed progressive restructuring of the scalp microbiome, becoming most evident by Day 28. This shift was characterized by a descriptive increase in the relative abundance of Cutibacterium acnes together with a numerical decline in Staphylococcus capitis, resulting in an increase in the C. acnes/Staphylococcus ratio from 2.83 to 4.51. Longitudinal compositional analyses further identified several bacterial genera associated with the temporal shift, including Streptococcus, Flavobacterium and Sphingomonas. The fungal community responded earlier to treatment, showing directional reduction of Malassezia-associated dominance together with enrichment of several non-Malassezia genera. Fungal richness increased significantly by Day 14 and remained elevated at Day 28, while beta-diversity analyses confirmed significant temporal restructuring of both bacterial and fungal communities.

Use of a piroctone olamine shampoo was associated with significant clinical improvement and measurable shifts in both bacterial and fungal scalp microbiota. These findings suggest that dandruff improvement may coincide with ecological restructuring of the scalp microbiome rather than broad microbial suppression.