Molecular Microbiology Research, 2025, Vol.15, No.1, 18-27 http://microbescipublisher.com/index.php/mmr 21 and also makes infections worse and even causes sepsis (Freire et al., 2022). The existence of these toxins makes E. coli very dangerous and sometimes even cause death. 4.3 Immune evasion strategies employed by pathogenic E. coli In order to stay in the human body for longer, pathogenic E. coli has developed some "hide-seek" skills. For example, they will make a protease called Pic. This enzyme can cut off complement protein and reduce the attack of the immune system (Abreu et al., 2015). The aforementioned Sat toxin also has a similar function, which also breaks down complement proteins, thereby helping bacteria escape and clearance (Figure 2) (Freire et al., 2022). These proteases not only act against the innate immune system, but also enhance the bacteria's ability to infect bacteria. Moreover, UPEC can also penetrate human cells and reproduce inside, which can also avoid many immune mechanisms. This shows that E. coli uses a complete set of complex methods to escape the human body's defense. Figure 2 Hypothetical model of sat enrollment in BSI and sepsis pathogenesis (Adopted from Freire et al., 2022) Image caption: A: Sat-producing E. coli present in the urinary tract or in the intestinal tract secretes Sat, causing cellular damage in the bladder, kidneys, or intestine. This damage allows the bacteria to move forward to the blood vessels, whose endothelial cells are also susceptible to Sat cytotoxic action; B: Vascular damage caused by Sat allows E. coli to access the bloodstream; C: Sat secretion in the bloodstream protects E. coli from complement killing, by direct cleavage of C2, C3, C3b, C4, C4b, C5, C6, C7, C8 and C9. This evasion mechanism allows Sat-producing E. coli to multiply and disseminate in the bloodstream and consequently reach other organs such as the spleen and the liver, where new sites of infection can be established and a magnification of the immune response can occur, facilitating the progression of the disease to sepsis (Adopted from Freire et al., 2022) 5 Environmental and Host Factors Influencing Pathogenicity 5.1 Influence of the host’s immune system on E. coli pathogenicity The body's immune system plays a very important role in fighting E. coli. Under normal circumstances, E. coli and the immune system are "peaceful coexist", and their interactions will help the immune system stay alert. This slight stimulation prepares the body in advance to fight invading bacteria (Sarowska et al., 2019). But if the bacteria in the intestine loses balance, that is, "fungal disorder", the immune system may be "misleading". It may be overreacting or inadequate, which increases the risk of some diseases such as inflammatory bowel disease or multiple sclerosis (Petersen and Round, 2014). In this case, pathogenic E. coli is more likely to take advantage of the situation and cause infection.
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