Isolation of a surfactin-producing strain of Bacillus subtilis and evaluation of the probiotic potential and antioxidant activity of surfactin from fermented soybean meal

BACKGROUND

Surfactin, usually produced by microbial metabolism, has many advantages including low toxicity, high biodegradability, and stability at extreme pH levels and temperatures, making it suitable for industry. However, its commercial production has not yet been achieved.

RESULTS

A strain with a strong surfactin-producing ability was isolated and identified as Bacillus subtilis SOPC5, based on the appearance of colonies, microscopic observation, and 16S rDNA sequencing. The isolate exhibited significant tolerance to acid, bile, gastric, and intestinal juices, and was sufficiently susceptible to antibiotics. Bacillus subtilis SOPC5 showed high levels of auto-aggregation and surface hydrophobicity, and a strong capacity to secrete protease, amylase, and cellulase. The strain also exhibited antibacterial activity against Staphylococcus aureus 10 306 with a antibacterial circle diameter of 18.0 ± 1.1 mm. The maximal yield of surfactin (1.32 mg mL-1) was obtained by fermenting soybean meal (SBM) using the isolate under the following conditions: SBM 86 g L-1, inoculation 1.5 × 107 CFU mL-1, FeSO4 1.2 mg L-1, MnSO4 2.6 mg L-1, MgSO4 0.5 mg mL-1, L-Glu 4 mg L-1, temperature 33 °C, duration 120 h, and shaking at 210 rpm. The purity of surfactin was 97.42% as measured by high-performance liquid chromatography (HPLC). The half inhibitory concentration (IC50) values for surfactin to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS·+) were 1.275 ± 0.11 and 0.73 ± 0.08 mg mL-1, respectively.

CONCLUSION

This study provides a scientific basis for the application of B. subtilis SOPC5 (as a potential probiotic) and the preparation of its metabolic product (surfactin). © 2024 Society of Chemical Industry.

Safety Evaluation and In vivo Strain-Specific Functionality of Bacillus Strains Isolated from Korean Traditional Fermented Foods

Unveiling and understanding differences in physiological features below the species level may serve as an essential fast-screening tool for selecting strains that can promote a specific probiotic effect. To study the intra-species diversity of Bacillus, a genus with a wide range of enzyme activities and specificity, 190 Bacillus strains were isolated from traditional Korean fermented food products. Altogether, in the preliminary safety screening, 8 of these strains were found negative for lecithinase and hemolysis activity and were selected for further investigations. On the basis of different levels of enzyme functionalities (high or low proteolytic, amylolytic, and lipolytic (PAL) activities), two Bacillus subtilis strains were selected for an in vivo study. Each of the two strains was separately administered at a level of 1 × 10⁸ CFU per day to C57BL/6 mice that were fed 60% high-fat diet ad libitum for 8 weeks, while Xenical, an anti-obesity drug, was used as a positive control in the experimental setup. B. subtilis M34 and B. subtilis GS40a with low and high amylolytic activities, respectively, induced significantly different and contrasting physiological effects. The production of short-chain fatty acids appeared to be closely associated with a shift in the gut microbiota.