Functional Properties of Heat-Killed Lactic Acid Bacteria Isolated from Vietnamese Feces

This study evaluated the functional properties of lactic acid bacteria (LAB) isolated from Vietnamese feces under various heat-treatment conditions by assessing their antioxidant activity, lipoteichoic acid (LTA) content, and immunomodulatory effects. Among the six LAB strains isolated, four (Ligilactobacillus salivarius V4, Limosilactobacillus fermentum V8, Lactiplantibacillus plantarum V10, and V11) were selected based on their acid and bile resistance, proteolytic activity, and β-galactosidase activity. Heat treatment (65°C for 60 min or 95°C for 10 min) completely inactivated LAB, reducing protein and phosphate content as treatment intensity increased. Antioxidant assays [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), 2,2-diphenyl-1-picrylhydrazyl, and ferric reducing antioxidant power] revealed comparable or superior activity in heat-treated (HT) cells compared to live cells, with L. plantarum V10 showing the highest activity. LTA content analysis of HT L. plantarum V10 showed the highest LTA content (1,244 ppm) at 65°C for 60 min, surpassing that of the commercial dead cell Enterococcus faecalis EF-2001. Anti-inflammatory activity tests indicated no cytotoxicity at concentrations below 107 CFU/mL, and HT cells treated at 65°C for 60 min showed the highest inhibition of nitric oxide and the lowest expression levels of inflammatory cytokines [tumor necrosis factor-α, interleukin (IL)-1β, IL-6]. Consequently, 65°C for 60 min was established as the optimal heat-treatment condition. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis analysis following digestive enzyme treatment revealed that HT L. plantarum V10 was not degraded by amylase or pepsin, but was partially degraded by oxgall. These findings suggest that HT L. plantarum V10 is a promising functional ingredient with antioxidant and immunoregulatory properties and is especially suitable for the Southeast Asian market.