Short‐Term Promotion of Beneficial Skin Commensal <i>Staphylococcus</i> Species Growth by PS‐B1, a Postbiotic Composed of Lactic Acid Bacteria‐Fermented Soy Milk

Postbiotics, defined as non-viable microbial cells, their components, or metabolites, are emerging functional ingredients that can modulate host health, including skin barrier function, immune responses, and microbial balance. The human skin microbiome plays a critical role in maintaining skin health, and the selective stimulation of beneficial commensals is of growing interest for cosmetic applications. PS-B1, a postbiotic derived from lactic acid bacteria during soy fermentation, is already used in beverages and cosmetics, but its direct effects on skin-resident bacteria have not been fully elucidated. In this study, we evaluated the effects of preservative-containing and preservative-free PS-B1 on the growth of twelve Staphylococcus strains and seven Cutibacterium acnes strains in vitro. The preservative-containing PS-B1 showed strong growth inhibition for all strains at higher concentrations. In contrast, the preservative-free PS-B1 significantly promoted the growth of Staphylococcus epidermidis, Staphylococcus hominis, and Staphylococcus lugdunensis, while Staphylococcus warneri was the only strain exhibiting growth inhibition. Other species showed no notable changes. Additionally, all C. acnes strains displayed slight growth enhancement. These results indicate that preservative-free PS-B1 selectively stimulates beneficial skin commensals, particularly S. hominis, revealing a previously unrecognized potential of PS-B1 to support a balanced skin microbiome.