Lactobacillus-derived extracellular vesicles as postbiotic modulators of redox signalling and cellular senescence in skin homeostasis

: The skin acts as a dynamic barrier-combining physical, chemical, and immunological defences-while hosting a diverse microbiome essential for cutaneous homeostasis. Dysbiosis and impaired redox balance are linked to various inflammatory conditions; however, the mechanisms by which microbial signals regulate skin cell senescence remain unclear. This study evaluated the effects of Lactobacillus-derived extracellular vesicles (EVs) on human keratinocytes (HaCaTs) and dermal fibroblasts (HDFs) under both physiological and stress-induced premature senescence (SIPS) conditions. SEM analysis confirmed the presence of spherical membrane-bound structures consistent with EV morphology, providing a qualitative characterisation. Functional assays indicate that low concentrations of EVs (1.25-2.5% v/v) increase the metabolic activity of HaCaT cells; however, only the 1.25% v/v concentration significantly promotes early wound closure, whereas the 2.5% v/v concentration induces a decoupling between metabolism and motility. In HDFs, EVs significantly reduced basal intracellular reactive oxygen species (ROS) levels, demonstrating an inherent capacity to modulate redox homeostasis. Furthermore, immunofluorescence analysis revealed that EVs exert cell-specific and context-dependent modulatory effects on the DNA damage response (DDR) and senescence-associated secretory phenotype (SASP). While EVs significantly regulated p21, 53BP1, and MMP-3 expression across both cell types under both basal and SIPS conditions, a specific modulation of COX-2 was observed exclusively in fibroblasts following SIPS induction. These results indicate that Lactobacillus-derived EVs support epidermal regeneration and protect dermal cells from molecular senescence by modulating redox-sensitive pathways. Our findings highlight the potential of these vesicles as multifunctional postbiotic regulators-providing a mechanistic basis for future strategies aimed at maintaining skin homeostasis and mitigating cellular ageing.