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 (<i>Ligilactobacillus salivarius</i> V4, <i>Limosilactobacillus fermentum</i> V8, <i>Lactiplantibacillus plantarum</i> V10, and V11) were selected based on their acid and bile resistance, proteolytic activity, and <i>β</i>-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 <i>L. plantarum</i> V10 showing the highest activity. LTA content analysis of HT <i>L. plantarum</i> V10 showed the highest LTA content (1,244 ppm) at 65°C for 60 min, surpassing that of the commercial dead cell <i>Enterococcus faecalis</i> 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-<i>α</i>, interleukin (IL)-1<i>β</i>, 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 <i>L. plantarum</i> V10 was not degraded by amylase or pepsin, but was partially degraded by oxgall. These findings suggest that HT <i>L. plantarum</i> V10 is a promising functional ingredient with antioxidant and immunoregulatory properties and is especially suitable for the Southeast Asian market.