# Integration of _Salmonella_ into Combination Cancer Therapy
## 4 Clinical Application of _Salmonella_ in Combination Cancer Therapy
### Challenges  
Despite the success reported with the use of _Salmonella_ as an integral part of combination therapies, the translation of this success from pre-clinical to human clinical application still poses major challenges. The limitations associated with the clinical application of combinatorial therapies mirror those related to the implementation of _Salmonella_ therapy alone. Although BMCT dates back to the mid-19 th century, the introduction of _Salmonella_ to human cancer treatment remains pending due to several challenges, including:  
* **Clinical safety.** Bacterial pathogens can cause diseases through the action of virulence factors. On the other hand, the attenuation of virulence factors has been correlated with decreased anti-tumor therapeutic effects [183; 184]. The optimized balance between reduced virulence and clinical efficacy remains the major challenge in the clinical application of BMCT. Moreover, not all bacterial strains that showed success in preclinical models can be used in clinical settings due to their distinct pathologies that differ in animals and humans.
* **Route of administration.** The systemic administration of bacteria increases the risk of toxicity and potential adverse effects of the infection. The oral administration is considered relatively safer but at the expense of therapeutic efficacy.
* **Dose optimization.** Since live bacteria proliferate in target tissues, the effective dose does not necessarily mirror the administered dose. The effective dose is dependent on different factors including the route of administration, accessibility to target tissues, level of vascularization, tumor immunogenicity, and the presence of tumor-infiltrating inflammatory cells [21].
* **Genetic instability.** Live genetically engineered bacteria that carry antibiotic resistance genes or mobile genetic elements are not suitable for clinical use since these recombinant elements can mediate horizontal gene transfer [185]. Other recombinant plasmids can be lost or mutated before reaching tumor tissues, leading to exaggerated infection or therapeutic failure [186].
* **Bacterial growth control _in vivo_.** The timely elimination of bacteria using an antibiotic intervention is critical since the early administration of antibiotic may eliminate the infection before an anti-tumor effect has been achieved, whereas a late intervention would result in unpredictable systemic inflammatory response. Microbiota disturbance and the development of antibiotic resistance should be kept in mind when using antibiotics to manage bacterial growth.
* **Tumor recurrence.** Despite the considerable role of bacteria in treating cancer, subsequent tumor recurrence could still be possible [14]. This may be attributed to the activation of immune tolerance or evasion mechanisms that interfere with the bacteria-mediated immune response [25].
* **Patient selection.** Chemotherapeutic agents suppress the immune system and interfere with delivering the immunomodulatory effects induced by BMCT. Therefore, the risk of bacterial infection is substantially increased in chemotherapy-receiving patients. In addition, brain abscesses, diverticulitis, or recent radiation might enhance the unintentional growth of bacteria in non-target tissues. Live bacteria also have the potential to colonize the foreign bodies in patients with joint replacement, artificial heart valves, and impanated medical devices [22]. Pre-exposure and anti-bacterial immunity should be also taken into account since they might interfere with the anti-tumor immune-stimulatory effects of the bacteria and result in treatment failure.