Abstract
This study evaluated the antibacterial effects of cell-free supernatants (CFS) produced by selected lactic acid bacteria (LAB) strains isolated from Tibetan Kefir Grains against Alicyclobacillus acidoterrestris. Among 17 isolated LAB strains, Lactobacillus paracasei strain MRS-4 exhibited the strongest inhibition effect on A. acidoterrestris, with an inhibitory diameter of 18.03 mm when assayed with the oxford cup method. The minimum inhibitory concentration and the minimum bactericidal concentration values of CFS from Lactobacillus paracasei strain MRS-4 against vegetative A. acidoterrestris cells were both 125 mg/mL. The CFS exhibited good thermal stability, but its antibacterial activity depended on pH, which decreased after protease treatment. Based on LC-MS analysis, organic acids were the most prevalent components of the CFS. These results indicate that the inhibition of L. paracasei strain MRS-4 against the growth of A. acidoterrestris was mainly due to the production of organic acids. Treatment of A. acidoterrestris with CFS resulted in DNA and protein leakage from cells, with scanning electron microscopy demonstrating that A. acidoterrestris cells suffered serious morphological damage after CFS treatment. Collectively, these results indicate that CFS from L. paracasei strain MRS-4 exerts excellent antibacterial activity against A. acidoterrestris cells by destroying their cellular integrity.
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