Inhibitory mechanism of cell-free supernatants of Lactobacillus plantarum on Proteus mirabilis and influence of the expression of histamine synthesis-related genes

The purpose of this study was to evaluate the inhibitory mechanism of cell-free supernatants of Lactobacillus plantarum (LCFS) on Proteus mirabilis and estimate the expression of histamine synthesis-related genes. The minimum inhibitory concentration (MIC) of the LCFS and antibacterial curve assay was determined by the tube double-dilution method. The antibacterial mechanism was explored by cell integrity, morphology, and structure of P. mirabilis. Further, the expression of the histidine decarboxylase (hdcA) gene and histidine/histamine antiporter (hdcP) gene of P. mirabilis and the intracellular and extracellular contents of histidine and histamine were determined. The MIC of the LCFS was 12.5 mg/mL, and the antibacterial curve assay showed an inhibited growth of P. mirabilis. The increase in the Alkaline Phosphatase (AKP) content, membrane permeability, and potential indicated the destruction of the cytoplasm, cell membrane, and cell wall of P. mirabilis. The results of SEM and TEM revealed that the LCFS disrupted the normal morphology and structure of the cell. Moreover, differences in the extracellular and intracellular histidine and histamine content between the MIC group and the control group was consistent with the decreased expression of hdcA and hdcP genes in the MIC group observed in the previous antimicrobial studies. Hence, the inhibitory mechanism of LCFS on histamine formation was clarified at cellular and molecular levels.