# A Review of the Use of Native and Engineered Probiotics for Colorectal Cancer Therapy
## 6 Conclusions and Future Perspectives  
To date, considerable progress in the prelimic has been made in CRC therapy based on engineered bacteria. However, there are no reports of clinical trials, and several questions have yet to be addressed before bacteria can be applied in the clinic. Several reviews have discussed the limitations of probiotic-mediated CRC therapy [8], including systemic infections, deleterious metabolic activities, excessive immune stimulation in susceptible individuals, and resistance gene transfer; this study mainly focuses on the mechanism of probiotic-mediated CRC therapy [15] and the delivery of therapeutic payloads using engineered bacteria for improving the therapeutic efficacy of CRC. Nonetheless, many issues need to be addressed before engineered bacteria can be used clinically. Although the toxicity of bacteria can be attenuated with gene engineering, any residual toxicity may be fatal for advanced patients with poor immunity. In addition, the use of bacteria as anticancer agents for patients with clinical cancer and their toxicity and side effects require rigorous verification by different clinical trials and long-term follow-up. Similarly, the use of low doses of bacteria to exert excellent therapeutic effects should be the focus of attention.  
The genetic instability of engineered bacteria arises from the potential loss and mutation of the recombinant plasmid carried within the bacterial host over successive generations, particularly in the absence of antibiotic pressure. This instability poses various risks, including treatment failure and increased infection severity. One potential solution is to modify the bacterial genome directly to achieve the desired outcome. However, this approach necessitates careful consideration of how genomic alterations might impact bacterial survival. Achieving a stable inheritance of engineered bacteria within the host remains a significant challenge, indicating the need for further research and development in this area.  
Regarding the biosafety of bacterial administration, bacteria are living organisms, and they may mutate in patients and proliferate in the human body. The occurrence of septicemia will be another disaster for patients. As such, the clinical development of bacterial therapy faces substantial hurdles due mainly to the potential adverse effects of infection. Furthermore, unlike other small molecules or clinical agents, live bacteria cannot be sterilized by filtration or heating. The kind of liquid used for bacterial dilution, the conditions to be used, and their effect on bacterial activity are problems in clinical translation as well. In addition, significant systemic infection with bacteria and the risk of toxicity by injection exert a psychological burden on patients, which results in decreased treatment compliance by patients. Therefore, the oral administration of bacteria without affecting efficacy may be an optimal approach.  
Regarding bacterial clearance in vivo, after the tumor has subsided, suitable antibiotics can be used to kill bacteria in the host. However, the amount of antibiotics, the resistance of the host, and the destruction of balance in the host's gut microbiota are new problems that need to be solved. Moreover, antibiotic abuse presents a significant global challenge. The excessive usage of antibiotics hastens the proliferation of drug-resistant bacteria.  
Conceptualization and structure, H.H. and A.K.; writing--original draft preparation, Y.Z. and H.T.; writing--review and editing, T.Z. and A.K.; funding acquisition, H.H. All authors have read and agreed to the published version of the manuscript.  
The authors thank the National Natural Science Foundation (Grant No: 32200080; 32070117).  
The authors declare no conflicts of interest.