Postbiotics and Nicotinamide Utilize Distinct Mechanisms to Improve Skin Barrier Integrity, Inflammation, and Keratinocyte Differentiation

The modulation of immune responses and tissue regeneration by postbiotics is a rapidly advancing area in skin care. Here, we show that whole-cell postbiotics derived from Bifidobacterium breve, Limosilactobacillus reuteri, and Ligilactobacillus salivarius, along with nicotinamide (NAM), enhance keratinocyte growth, differentiation, and skin epithelial barrier integrity in ex vivo human skin, as determined by electrical impedance spectroscopy (EIS), multiomics, and machine learning. B. breve promoted keratinocyte differentiation and suppressed inflammatory pathways, while L. reuteri and L. salivarius primarily reduced inflammatory pathways. Although NAM downregulated keratinocyte differentiation, it exerted anti-inflammatory effects. Machine learning analyses linked EIS changes to certain genes, highlighting strain-specific mechanisms. In addition, B. breve, L. reuteri, and NAM mitigated a common skin cleanser-induced skin epithelial damage, further supporting their therapeutic potential. In conclusion, integrating skin barrier measurements with omics and machine learning enabled the dissection of essential anti-inflammatory and keratinocyte differentiation mechanisms and genes of a strengthened skin barrier.