In the respiratory tract and lung tissue, a balanced physiological response is essential for Actinobacillus pleuropneumoniae to survive various types of challenges. ClpP, the catalytic core of the Clp proteolytic complex, is involved in various stresses response and regulation of biofilm formation in many pathogenic bacteria. To investigate the role of ClpP in the virulence of A. pleuropneumoniae, the clpP gene was deleted by homologous recombination, resulting in the mutant strain S8ΔclpP. The reduced growth of S8ΔclpP mutant at high temperatures and under several other stress conditions suggests that the ClpP protein is required for the stress tolerance of A. pleuropneumoniae. Interestingly, we observed that the S8ΔclpP mutant exhibited an increased ability to take up iron in vitro compared to the wild-type strain. We also found that the cells without ClpP displayed rough and irregular surfaces and increased cell volume relative to the wild-type strain using scanning electron microscopy (SEM). Confocal laser scanning microscopy (CLSM) revealed that the S8ΔclpP mutant showed decreased biofilm formation compared to the wild-type strain. We examined the transcriptional profiles of the wild type S8 and the S8ΔclpP mutant strains of A. pleuropneumoniae using RNA sequencing. Our analysis revealed that the expression of 16 genes was changed by the deletion of the clpP gene. The data presented in this study illustrate the important role of ClpP protease in the stress response, iron acquisition, cell morphology and biofilm formation related to A. pleuropneumoniae and further suggest a putative role of ClpP protease in virulence regulation.
Previously, we have shown that glutathione can protect Lactococcus lactis against oxidative stress and acid stress. In this study, we show that glutathione taken up by L. lactis SK11 can protect this organism against osmotic stress. When exposed to 5 M NaCl, L. lactis SK11 cells containing glutathione exhibited significantly improved survival compared to the control cells. Transmission electron microscopy showed that the integrity of L. lactis SK11 cells containing glutathione was maintained for at least 24 h, whereas autolysis of the control cells occurred within 2 h after exposure to this osmotic stress. Comparative proteomic analyses using SK11 cells containing or not containing glutathione that were exposed or not exposed to osmotic stress were performed. The results revealed that 21 of 29 differentially expressed proteins are involved in metabolic pathways, mainly sugar metabolism. Several glycolytic enzymes of L. lactis were significantly upregulated in the presence of glutathione, which might be the key for improving the general stress resistance of a strain. Together with the results of previous studies, the results of this study demonstrated that glutathione plays important roles in protecting L. lactis against multiple environmental stresses; thus, glutathione can be considered a general protectant for improving the robustness and stability of dairy starter cultures.Lactic acid bacteria (LAB) are widely used in the food and fermentation industries (5). During the manufacturing process, LAB often encounter various environmental stresses, including acid stress, oxidative stress, heat stress, and cold stress. The physiological responses of LAB to these environmental stresses have been extensively studied and characterized (10,16,20,24,25,35,42,45,47). In addition, LAB are often challenged with high salt concentrations during cheese fermentation (11), vegetable pickling (39), food preservation (17), and fermentative production of lactic acid (4). The osmotic stress triggered by high salt concentrations results in both structural and physiological injury of cells (8,40). Therefore, the ability of LAB to survive, grow, and metabolize actively under osmotic stress conditions is very important in industry (53).Bacteria have evolved various mechanisms to survive osmotic challenge. When there is a hyperosmotic shock, some bacteria (e.g., Escherichia coli, Klebsiella pneumoniae, and Lactococcus lactis) exhibit plasmolysis due to a rapid efflux of water to balance the change in osmotic pressure (8, 9). The accumulated ions are then exchanged by osmoprotectants, such as amino acids (e.g., glutamate [8,44] and proline [8,9,36]) and quaternary amines (e.g., glycine betaine [36,37,48]). Various proteins contribute to the accumulation of osmoprotective substances during osmotic stress (8,9,36,37 (27). Several osmotic stress-specific proteins, including enzymes involved in proline biosynthesis in B. subtilis, glucosylglycerol biosynthesis in Synechocystis sp., and glycerol biosynthesis in A. nidulans, have been identifi...
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