Efficiency of Conditionally Attenuated Salmonella enterica Serovar Typhimurium in Bacterium-Mediated Tumor Therapy

Increasing numbers of cancer cases generate a great urge for new treatment options. Applying bacteria like Salmonella enterica serovar Typhimurium for cancer therapy represents an intensively explored option. These bacteria have been shown not only to colonize solid tumors but also to exhibit an intrinsic antitumor effect. In addition, they could serve as tumor-targeting vectors for therapeutic molecules. However, the pathogenic S. Typhimurium strains used for tumor therapy need to be attenuated for safe application. Here, lipopolysaccharide (LPS) deletion mutants (Delta rfaL, Delta rfaG, Delta rfaH, Delta rfaD, Delta rfaP, and Delta msbB mutants) of Salmonella were investigated for efficiency in tumor therapy. Of such variants, the Delta rfaD and Delta rfaG deep rough mutants exhibited the best tumor specificity and lowest pathogenicity. However, the intrinsic antitumor effect was found to be weak. To overcome this limitation, conditional attenuation was tested by complementing the mutants with an inducible arabinose promoter. The chromosomal integration of the respective LPS biosynthesis genes into the araBAD locus exhibited the best balance of attenuation and therapeutic benefit. Thus, the present study establishes a basis for the development of an applicably cancer therapeutic bacterium. IMPORTANCE Cancer has become the second most frequent cause of death in industrialized countries. This and the drawbacks of routine therapies generate an urgent need for novel treatment options. Applying appropriately modified S. Typhimurium for therapy represents the major challenge of bacterium-mediated tumor therapy. In the present study, we demonstrated that Salmonella bacteria conditionally modified in their LPS phenotype exhibit a safe tumor-targeting phenotype. Moreover, they could represent a suitable vehicle to shuttle therapeutic compounds directly into cancerous tissue without harming the host.