Bt Research 2025, Vol.16, No.3, 110-117 http://microbescipublisher.com/index.php/bt 113 energy to coping with stress (Guo et al., 2020). At low temperatures, the expression of cold shock proteins decreases instead, suggesting that low temperatures may inhibit the synthesis of certain stress proteins. 4.2 PH stress Under alkaline conditions, Bt can up-regulate genes related to glycolysis and organic acid synthesis, such as the enzyme that catalyzes the conversion of α -ketoglutarate to malic acid. These changes enable it to produce more organic acids (such as malic acid, lactic acid), thereby regulating intracellular and extracellular pH and enhancing adaptability. More than twenty glycolytic pathway genes were found to be upregulated, indicating that the role of carbon metabolism in pH stress is very crucial. 4.3 Oxidative stress Under oxidative stress (such as H2O2), Bt up-regulates antioxidant enzymes (APX, SOD) and heme synthesis genes (hemH1, hemH2), thereby enhancing antioxidant defense capabilities. The iron uptake regulatory protein Fur has a bidirectional effect on hemH1 and hemH2. It can both promote and inhibit, helping to regulate heme synthesis, heavy metal detoxification and oxidative stress tolerance, and maintain homeostasis. Furthermore, long-term infection experiments have shown that in the host, non-spore cells of Bt activate oxidative stress genes and slow metabolism simultaneously, which can maintain long-term survival (Toukabri et al., 2022). 4.4 Osmotic stress Under salt stress, salt-tolerant Bt strains (such as PM25) can increase the expression of antioxidant enzyme genes like APX and SOD. Meanwhile, they synthesize more osmotic regulatory substances, such as soluble sugars and proline, to enhance tolerance. Proteomics studies have found that as salt concentration increases, related proteins such as transferases and hydrolases increase, while some toxins and cold shock proteins decrease. This indicates that Bt responds to high osmotic pressure by regulating secretory proteins and metabolic networks (Subramanian et al., 2021). 4.5 Nutrient limitation When there is malnutrition, whether in the host or in the environment, Bt downregulates movement-related genes and up-regulates biofilm and nutrient acquisition-related genes, thereby entering a metabolic slowdown state to prolong survival (Toukabri et al., 2022). During the Necrotrophism stage, it activates hydrolase, protease and oligopeptide transporter genes, and utilizes host resources in a short period of time. Subsequently, Bt enters a persistent state and relies on stress-related genes to maintain long-term survival. 5 Regulatory Networks and Stress-Responsive Pathways 5.1 Two-component systems (e.g., PhoP/PhoR, DegU/DegS). The two-component system is the main way for bacteria to sense external signals and respond. In Bt, the specific mechanisms of systems such as PhoP/PhoR and DegU/DegS have not been fully studied. However, existing studies have shown that similar systems can regulate iron uptake, oxidative stress and metabolic pathways. For instance, the iron uptake regulatory protein Fur can sense changes in iron ion concentration. It can directly bind to the promoters of heme synthase genes hemH1 and hemH2, thereby controlling heme synthesis, heavy metal detoxification and antioxidant defense, and helping to maintain cellular homeostasis. In addition, the Rap-Phr type of quorum sensing system is also involved in regulating functions such as spore formation, biofilm and group behavior (Gastelum et al., 2019). 5.2 Sigma factors (σB, σH, σE) in stress adaptation. The σ factor is a key factor in bacterial transcriptional regulation. In Bt, σ^54 (σL) can regulate the expression of genes related to nitrogen metabolism, carbon metabolism and various stresses, so that cells can better adapt to changes in nutrition and environment (Peng et al., 2015). The σ factor can also work with some specific regulatory proteins, such as AcoR, to control metabolic pathways such as acetoacetic acid decomposition to adapt to different conditions (Peng et al., 2020). The synthesis of cold shock proteins and heat shock proteins is also controlled by the σ factor and can play a protective role under temperature changes and osmotic pressure stress.
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