# A Review of the Use of Native and Engineered Probiotics for Colorectal Cancer Therapy
## 1 Introduction  
Colorectal cancer (CRC) is the most commonly diagnosed cancer and the leading cause of death worldwide [1]. Mutations in specific genes, including oncogenes, tumor suppressor genes, and genes associated with DNA repair mechanisms, are responsible for the development of CRC [2]. At present, the choice of CRC treatment measurement is generally based on tumor-related characteristics and patient information [2]. Chemotherapeutic drugs (5-Fu and oxaliplatin) and targeted drugs (ectuximab and bevacizumab), as the first choice for treatment for patients with advanced CRC, can significantly improve patient survival. These drugs have systemic administration-related toxic side effects in the treatment of the CRC. and may lead to a weaker treatment response due to the heterogeneity of CRC [3, 4]. Meanwhile, the relative lack of new specific biomarkers also hinders the development of more effective targeted drugs [5]. Therefore, it is critical to develop effective and alternative methods for CRC treatment. The gut microbiota is closely related to the host's health. CRC pathogenesis is often accompanied by intestinal microbial dysbiosis. Increased abundances of multiple gut bacteria, including _Fusobacterium nucleatum_ and _Peptostreptococcus anaerobius_, promotes colorectal carcinogenesis [6, 7]. Probiotic bacterial strains can restore gut microbial homeostasis via the modulation of gut microbiota [8, 9]. The prevention or treatment of CRC using probiotics has become a potential strategy [8]. Similarly, the use of oncolytic bacteria such as _Salmonella typhimurium_ have also been researched as a CRC treatment strategy [10, 11, 12]. This strain induces tumor regression through various mechanisms, including the direct induction of apoptosis; the depletion of nutrients and oxygen; and the activation of thetumor-specific immune response [13]. However, wild bacteria have weak therapeutic ability and uncontrolled physiological behavior. The characteristics of simple metabolic activity and single therapeutic function make it difficult to cope with malignancy efficiently [14]. Meanwhile, viable microorganisms may cause unpredictable harm to the host, including infections, inflammatory effects, and carcinogenesis [15; 16]. Therefore, wild-type bacteria are still far from being an ideal tumor therapeutic agent. With great advancements in synthetic biology and biomedical engineering, the therapeutic benefits of wild-type bacteria have been enhanced via directional genetic modifications. The anti-tumor ability of bacteria is further improved. Genetically modified bacteria can be employed as live vectors to deliver various therapeutic payloads, including cytotoxic proteins, immunoregulatory factors, prodrug-converting enzymes, angiogenesis regulation proteins, RNA interference (RNAi) molecules, and immunoregulatory factors [17; 18]. Meanwhile, the development of bacterial-component-based strategies has also been applied to CRC therapy, such as outer engineered membrane vesicles (OMVs), which are nanosized vesicles with excellent tumor penetrability [19]. In addition, the targeting of genetically modified bacteria is further optimized for precise intervention at tumor lesion sites. One strategy involves enhancing the capacity of bacteria to bind to tumor cells. For instance, bacteria were genetically altered to produce the Histone-like protein A (HlpA), known for its strong affinity to the surfaces of colorectal cancer (CRC) cells, thereby enhancing targeting efficacy [20]. Another strategy involves the precise control of therapeutic payload expression and release. One typical example is the integration of inducible promoters into the gene expression system for the control of target gene expression at a specific location, which can effectively avoid the off-target delivery of toxic drugs [21]. Unlike other therapeutics, bacteria-mediated tumor therapy is not directly affected by the \"genetic makeup\" of the tumor [22], which makes it a unique tumor treatment strategy.