Evaluation of lactic acid-Producing Bacillus and Sporolactobacillus for antilisterial activity

Fifty-five strains of Bacillus (53 B. laevolacticus and 2 B. racemilacticus) and 31 strains of Sporolactobacillus (3 S. inulinus, 19 S. laevus, and 9 S. racemicus) were screened for specific inhibitory activity against 8 strains of Listeria monocytogenes. Putative producer strains were propagated in Eugon, lactobacillus MRS, or sporolactobacillus broths (48-96 h, 37°C, 9.4% CO₂). Assays to detect inhibition of Listeria monocytogenes were carried out on Eugon or Bacto-Agar at 37°C (9.4% CO ₂). Inhibition was absent in direct assays using spot-on-the-lawn techniques but was found in well diffusion assays, and in deferred assays using stabinoculation techniques. Cell-free supernatant fluids of cultured MRS broths demonstrated superior inhibition to cell-free supernatants from Eugon and sporolactobacillus broths. The inhibitory activity of cell-free supernatant fluids was lost after neutralization (0.5M NaOH) or by dialysis (10,000 Da exclusion limit). The observed inhibition was most likely due to production of lactic acid.