# Bacteria-Mediated Modulatory Strategies for Colorectal Cancer Treatment
## 2 Bacterotherapy in CRC Treatment
### Mechanisms Used in Bacterotherapy in CRC
#### 2.1.2 Bacterial Peptides
_Colicin_  
The aforementioned bacteriocin colicin is produced by _Enterobacteriaceae_ such as _E. coli_. It is assumed that colicins take cytotoxic action on various malignant cells through membrane hole formation, a non-specified DNase and RNase activity and inhibition of the murein synthesis [72]. In previous research, the inhibitory effect of Colicin E1, E3, A, U on growth of HT-29 CRC cells among other human cancer cell lines was demonstrated [73], whereby HT-29 cells reacted insensitive to Colicin E1, while Colicin A treatment provoked the highest cytotoxicity against HT-29 cells [74]. _Microcin_  
According to studies that were carried out to evaluate the impact of Microcin E492, secreted by gram-negative _Klebsiella pneumonia_, on different cancer cell lines such as HeLa, Jurkat, RJ2.25 and also CRC cells, its anti-tumor activity could be demonstrated. Hereby the main mode of action of Microcin E492 included pore formation in cancer cell membranes, finally leading to apoptosis by binding to Toll-like-receptor (TLR) 4 [75; 76]. Interestingly, inhibition of normal cells such as bone marrow cells, splenocytes, KG-1 or human tonsil cells has not been observed, demonstrating bacteria to selectively target malignant cells [75; 77]. _Pediocin_  
Another type of bacteriocin that has been shown to have lethal effects on CRC cells (HT-29 and DLD-1) probably due to its hydrophobic nature, is the bacterial toxin pedicoin, which is derived from _Pediococcus acidilactici_ (K2a2-3), a gram-positive bacterial strain that is able to grow in a wide range of pH, temperature and osmotic pressure. These treatments enable them to enter and grow in the intestinal tract [78]. The specific mechanism underlying pediocin's cytotoxic effect on cancer cells is not fully understood yet, but application of sequence alignment has demonstrated great homology between Pedicin PA-1 and Microcin E492. Since the latter has been studied in more detail and is known to interact with TLR4 for induction of apoptosis, Pedicoin PA-1 is also believed to interoperate with TLRs in order to initiate cell death [79]. _Phenazine_  
Phenazine displays another group of bacteriocin, nitrogen containing metabolites, such as phenazine 1-carboxylic acid and phenazine 1,6-di-carboxylic acid (PDC) that are secreted by many bacterial strains, including a remarkable number of _Pseudomonas_. Phenazines are crucial for biofilm synthesis and help to protect bacteria from competitive microorganisms because of their toxicity [80]. In previous studies, PDC secreted from _Pseudomonas aeruginosa_ was demonstrated to show substantial cytotoxic activity against CRC cells (HT29) among other cancer cell lines (HeLa, MCF7, DU145) in a dose-dependent manner. Thereby, its range of cytotoxic action was the greatest against colorectal HT29, HeLa and MCF7 [80; 81]. In addition, PDC was observed to negatively affect both cancer cell viability and DNA synthesis and to induce G1 cell cycle arrest, thus initiating apoptosis [81]. Azurin  
Azurin is another protein that is able to enter cancer cells and induce cell cycle arrest and apoptosis. The copper-containing protein found in _Pseudomonas aeruginosa_ and its peptide p28 have even been studied in clinical trials of phase 1 already that demonstrated p28's anti-cancer toxicity and safety [82; 83]. As a multi-targeting peptide, p28's anti-cancer activity is based on several mechanisms such as complex formation with p53 tumor suppressor, interferential action on the receptor of tyrosine kinase EphB2-mediated signaling process, reduced activity of VEGFR kinase, prevention of angiogenesis and interferential activity on P-cadherin protein expression [26; 84]. These findings were also supported by another study, where azurin was demonstrated to decrease CRC cell viability by apoptosis induction, whereas non-cancer cells remained unaffected. This finding further underlines the potential of azurin as a selective anti-cancer agent [85].