# Bacteria-Mediated Modulatory Strategies for Colorectal Cancer Treatment
## 2 Bacterotherapy in CRC Treatment
### Mechanisms Used in Bacterotherapy in CRC  
There have been several mechanisms of bacteria described to date that researchers are taking advantage of in the treatment of CRC via bacterotherapy, such as formation of pores in the cell membrane, inhibition of metastasis, tumor necrosis or apoptosis [20; 40]. In particular, apoptosis has been known as a key goal in cancer treatment for several decades in order to eradicate tumor tissue that is characterized by a loss of balance between cell proliferation and death [41; 42]. The term apoptosis or programmed cell death,describes a very complex process, which includes various pathways and mechanisms (Figure 1) [42; 43]. In general, it is distinguished between the intrinsic (mitochondrial-dependent) pathway and the extrinsic (receptor-dependent) pathway, both finally leading to Caspase activity and apoptosis. Programmed cell death, stimulated by the intrinsic pathway, is characterized by cytochrome c release from pro-apoptotic proteins (B-cell lymphoma 1-Bcl-1, Bcl-2-associated X protein-Bax, Bcl-2-Antagonist of Cell Death-Bad and BCL2 Antagonist/Killer 1-Bak)-stimulated mitochondria from the intermembrane space into the cytosol, subsequently forming the apoptosome complex together with Caspase-9, eventually leading to apoptosis. The extrinsic pathway on the other hand is stimulated by cell membrane death receptors such as Tumor necrosis factor receptor (TNF-R) binding to natural ligands, whereby initiator Caspase-8 is induced, which promotes cleavage of further downstream caspases, finally inducing apoptosis [41; 42; 44; 45]. Moreover, receptor-ligand binding induces several cellular responses, including the activation of NF-kB, which can activate pro-apoptotic proteins depending on the cellular context [44; 45]. However, the two main apoptotic pathways must not be considered separately, since previous research showed that they are linked with each other and metabolites of one pathway can have an impact on the other [45]. Figure 1: Schematic diagram showing the mechanisms of apoptosis triggered by bacterial peptides in cancer cells. (**A**) Bacterial toxins, secreted by various bacterial strains can cause apoptosis via the mitochondria-dependent pathway by causing cell injury, for example, by cell membrane pore formation. Induction of the intrinsic pathway leads to activation of pro-apoptotic proteins (Bcl-1, Bad, Bax, Bak), which in turn stimulates the release of cytochrome c molecules from the mitochondrial intermembrane space into the cytosol. Cytochrome c, together with Caspase-9 forms a complex called the “apoptosome”, finally stimulating executioner caspases (e.g., Caspase-3) leading to cancer cell apoptosis. (**B**) Bacterial proteins and peptides can have a modulatory impact on cytokines such as TNF-\(\alpha\), resulting in activation or blockage of NF-\(\kappa\)B. With suppression of NF-\(\kappa\)B, which stimulates anti-apoptotic proteins Bcl-2 and Bcl-\(\kappa\)L, which in turn regulates apoptosis by blocking cytochrome c release, pro-apoptotic Bax and Bak-proteins remain stimulated and apoptosis is induced. (**C**) Besides stimulating the intrinsic pathway of apoptosis, probiotics are capable of apoptosis induction through stimulation of the extrinsic receptor-dependent pathway. Here, so called cell death receptors, such as TNF-R, bind to natural ligands, whereby initiator Caspase-8 and -10 are activated to cleave further downstream caspases, such as Caspase-3, which in turn induces cell apoptosis [41; 42; 43; 44; 45]. Altogether, to make use of these mechanisms such as bacteria-induced apoptosis and metastasis suppression and to establish efficient therapy methods within using bacteria, it is important to meet several framework conditions such as maximum cytotoxicity against cancer cells with minimum cytotoxicity towards intact cell tissue and the ability to selectively attack carcinomas [20; 40].