Phenotypic Evolution of Therapeutic Salmonella enterica Serovar Typhimurium after Invasion of TRAMP Mouse Prostate Tumor

Abstract

Salmonella has been of interest in cancer research due to its intrinsic ability to selectively target and colonize within tumors, leading to tumor cell death. Current research indicates promising use of Salmonella in regular administrations to remove tumors in mouse models while minimizing toxic side effects. However, selection of mutants during such long-term tumor colonization is a safety concern, and understanding selection of certain phenotypes within a tumor is an important consideration in predicting the long-term success of bacterium-based cancer treatment strategies. Thus, we have made an initial examination of selected phenotypes in a therapeutic Salmonella enterica serovar Typhimurium population developed from an archival wild-type LT2 strain and intraperitoneally injected into a 6-month-old TRAMP (transgenic adenocarcinoma of mouse prostate) mouse. We compared the original injected strain to isolates recovered from prostate tumors and those recovered from the spleen and liver of non-tumor-bearing TRAMP mice through phenotypic assessments of bacteriophage susceptibility, motility, growth rates, morphology, and metabolic activity. Tumor isolate traits, particularly the loss of wild-type motility and flagella, reflect the selective pressure of the tumor, while the maintenance of bacteriophage resistance indicates no active selection to remove this robust trait. We posit that the Salmonella population adopts certain strategies to minimize energy consumption and maximize survival and proliferation once within the tumor. We find these insights to be nonnegligible considerations in the development of cancer therapies involving bacteria and suggest further examinations into the evolution of therapeutic strains during passage through tumors.

Bacteria in Cancer Therapy: Renaissance of an Old Concept

“…Nowadays, suitable strains have been generated. They are obtained either by in vitro or in vivo passaging or by targeted gene deletion using molecular techniques [ 37 – 40 ]. The first technique relies on the selective pressure when the bacteria adapt to a particular environment within the host.…”

Strains, Mechanism, and Perspective:Salmonella-Based Cancer Therapy

“…When the TRAMP prostate cancer mouse model was intraperitoneally injected with 1 × 10 7 CFU, the ratio of Salmonella counts were up to 100-fold greater in the TRAMP mouse prostate tumor masses versus the usual Salmonella reservoirs of the liver and spleen after 72 hours (Kazmierczak et al, unpublished data). Strain CRC2631 recovered from TRAMP mouse prostate tumors showed significant loss of wild-type motility and flagella, indicating phenotypic evolution of this therapeutic strain within the tumor environment [ 27 ]. Further optimization of CRC2631 is under way.…”

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

“…Many bacteria have been shown to target experimental solid tumors with high specificity and exert therapeutic effects on tumors. In this context, safety strains have been established by in vitro or in vivo selection ( 7 ). In addition, clinical trials have been initiated, although their success was ambiguous ( 8 – 10 ).…”