Truncal acne represents a biologically distinct manifestation of acne vulgaris, yet its fungal ecology remains incompletely characterized. Previous work using internal transcribed spacer 2 (ITS2) sequencing suggested that truncal acne is associated with altered fungal richness and Malassezia species composition; however, fungal marker choice may influence ecological inference, particularly in sebaceous skin dominated by Malassezia. In this study, we characterized the truncal skin mycobiome of patients with truncal acne and healthy controls using internal transcribed spacer 1 (ITS1) amplicon sequencing. Skin swabs were collected from the upper back, and fungal communities were analyzed using QIIME 2 with taxonomic assignment against the UNITE v10.0 database. Baseline acne–control differences and doxycycline-associated patterns were evaluated using alpha- and beta-diversity metrics and differential abundance analyses. Doxycycline-associated patterns were assessed using paired, within-patient pre- and post-exposure comparisons. ITS1 profiling demonstrated that truncal acne was associated with altered baseline fungal ecology compared with controls, characterized by reduced alpha diversity and ASV-level differences within Malassezia-dominated communities. Beta-diversity analyses showed substantial overlap between acne and control samples, indicating limited global separation. Following doxycycline exposure, fungal communities remained Malassezia-dominant and did not demonstrate uniform convergence toward control profiles; instead, species- and ASV-level differences were heterogeneous across individuals and exposure durations. Together with prior ITS2-based findings, these results underscore the importance of marker-dependent perspectives when interpreting fungal ecology in sebaceous skin.

Truncal acne significantly impairs quality of life yet remains underexplored relative to facial acne, particularly with respect to fungal ecology. The trunk represents a distinct cutaneous niche characterized by thicker epidermis, larger follicular units, and frequent occlusion, and harbors a high abundance of Malassezia species. In this study, we used internal transcribed spacer 2 (ITS2) amplicon sequencing to characterize the truncal mycobiome in patients with acne and in healthy controls and to compare fungal community features across doxycycline exposure groups. Although serial sampling was planned, seven participants contributed a single follow-up sample after doxycycline treatment, and only two participants contributed multiple follow-up samples sufficient for true within-subject longitudinal analyses; therefore, most analyses represent exposure-stratified cross-sectional comparisons rather than confirmed temporal change. At baseline, truncal acne lesions exhibited increased fungal richness and distinct community composition compared with controls. Acne lesions were more frequently enriched for Malassezia globosa, whereas healthy controls were dominated by M. sympodialis. Across doxycycline exposure groups, fungal communities remained Malassezia-dominant with substantial inter-individual variability. Doxycycline exposure was associated with partial and heterogeneous differences in Malassezia species composition without uniform normalization toward control profiles. Because only fungal sequencing was performed, bacterial–fungal interactions were inferred from prior literature and not directly measured. These findings indicate that truncal acne is associated with a distinct fungal community structure and highlight the need for integrated, longitudinal multi-omics studies to clarify treatment-associated microbial dynamics.