Liquids left after yeast or bacteria ferment, sold as essences and 'probiotic' skincare. Hydrating; beyond that, evidence is small and mostly manufacturer-run.
Ferment filtrates are the strained liquid from growing yeasts such as galactomyces or saccharomyces; lysates are broken-open, non-living bacteria such as bifida or lactobacillus. They contain amino acids, sugars, organic acids, vitamins and peptides that act as humectants and may support the skin barrier.
'Probiotic' skincare is a misnomer: these products don't contain live microbes. Whether lysates usefully shift the skin's own microbes is an open research question.
Evidence comes mainly from lab studies and trials run by the companies that sell these ingredients. A double-blind trial of a 10% bifida lysate cream in 66 women with reactive skin found less sensitivity and dryness after two months. A 12-month study of galactomyces products reported fewer wrinkles and spots but had no control group and was run by the manufacturer.
66 women with reactive skin used a cream with 10% bifida lysate or a control cream twice daily for 2 months; the lysate group had less skin sensitivity, better barrier resistance and less dryness. Run by a cosmetics company.
86 women used galactomyces ferment filtrate products twice daily for 12 months; hydration rose and wrinkle, spot and roughness measures improved, which the authors describe as reversing part of the decline they had tracked over 11 years. No control group; the authors were employees of or consultants to the manufacturer.