It has been reported that the human skin microbiome is the second largest after the intestine, with approximately 1,000 species [1]. Cutibacterium acnes is the major anaerobic bacteria, especially in sebum-rich tissues such as the face [2]. C. acnes enzymatically degrades triglycerides contained in sebum to fatty acids and glycerol; fatty acids exhibit antibacterial activity and glycerol is one of moisturizing components, thereby contributing to skin barrier function and moisturization. On the other hand, C. acnes is also considered to be a pathogen that causes inflammatory skin condition in some circumstances, such as acne vulgaris [3]. In recent studies, C. acnes has been taxonomically subdivided into distinct subspecies that exhibit unique genetic, metabolic, and morphological properties [4], [5], [6]. Certain subspecies are considered to be associated with the development of acne vulgaris, while others have potential as candidates for beneficial bacteria on human skin [7], [8], [9]. If the number of pathogenic subspecies (e.g. subspecies acnes: type I) increases on the human skin, the risk of developing acne may increase, and the prognosis may worsen [8], [10]. In contrast, an increase in beneficial subspecies (e.g. subspecies defendens and elongatum: type II and III) is expected to contribute to a healthier skin environment [8], [10].

It is well known that the skin microbiome is influenced by various factors, including aging, environmental conditions, and geographic location [1], [11]. In particular, several studies have reported that long-term use of cosmetics can alter the skin microbiome, as facial skin is regularly exposed to various cosmetic ingredients through daily makeup application [12], [13], [14], [15]. Cosmetics are composed of various ingredients, such as ethanol, glycerol, and surfactants, as usually described in their labels. However, the direct effects of individual cosmetic components on C. acnes at the subspecies level remain unclear. We believe that the investigation into the effects of the ingredients to skin inhabitants should provide valuable insights for the development of cosmetics designed to optimize the skin microbiome.

In this study, we used common raw materials such as ethanol, glycerol, a surfactant (Tween 80), and polyamines, which have been reported to be associated with anti-aging effects [16]. To assess their impact at the subspecies level of C. acnes, we conducted direct in vitro evaluations using representative strains from each subspecies.