Influence of infection route and virulence factors on colonization of solid tumors by Salmonella enterica serovar Typhimurium

Motility is critical for effective distribution and accumulation of bacteria in tumor tissue

Motile bacteria can overcome the penetration limitations of cancer chemotherapeutics because they can actively migrate into solid tumors. Although several genera of bacteria have been shown to accumulate preferentially in tumors, the spatiotemporal dynamics of bacterial tumor colonization and their dependence on bacterial motility are not clear. For effective tumor regression, bacteria must penetrate and distribute uniformly throughout tumors. To measure these dynamics, we used an in vitro model of continuously perfused tumor tissue to mimic the delivery and systemic clearance of Salmonella typhimurium strains SL1344 and VNP20009, and Escherichia coli strains K12 and DH5α. Tissues were treated for 1 hour with 10(5) or 10(7) CFU ml(-1) suspensions of each strain and the location and extent of bacterial accumulation were observed for 30 hours. Salmonella had 14.5 times greater average swimming speed than E. coli and colonized tissues at 100 times lower doses than E. coli. Bacterial motility strongly correlated (R(2) = 99.3%) with the extent of tissue accumulation. When inoculated at 10(5) CFU ml(-1), motile Salmonella formed colonies denser than 10(10) CFU/(g-tissue) and less motile E. coli showed no detectable colonization. Based on spatiotemporal profiles and a mathematical model of motility and growth, bacterial dispersion was found to be necessary for deep penetration into tissue. Bacterial colonization caused apoptosis in tumors and apoptosis levels correlated (R(2) = 98.6%) with colonization density. These results show that motility is critical for effective distribution of bacteria in tumors and is essential for designing cancer therapies that can overcome the barrier of limited tumor penetration.

Antibiotic control of tumor-colonizing Salmonella enterica serovar Typhimurium

Systemic administration of Salmonella enterica serovar Typhimurium (S. typhimurium) into tumor-bearing mice results in preferential colonization of tumors and causes shrinkage and sometimes complete tumor clearance. However, in spite of these beneficial antitumor effects, the systemic administration of a bacterial pathogen raises serious safety concerns as well. Addressing those concerns, here, we demonstrate that tumor-colonizing Salmonella can be readily controlled by systemic administration of the antibiotic - ciprofloxacin. Treatment was most effective when started early postinfection. This was achieved at the expense of the efficacy of tumor therapy. In many of the mice treated in such a way, tumors re-grew again. Nevertheless, some mice were able to clear the tumor despite the start of antibiotic treatment only 24 h after the start of infection. Furthermore, we could demonstrate that such mice had elicited a specific antitumor immune response. Thus, S. typhimurium-mediated tumor therapy might be applied safely when combined with early antibiotic treatment. However, the therapeutic power of the bacteria needs to be enhanced in order to provide a more effective therapeutic tool.