Attenuated Salmonella typhimurium, a motile, nonpathogenic facultative anaerobic bacterium, has been demonstrated experimentally as a novel anticancer agent because of its favored growth within tumors. Specificity towards tumors allows for its potential use as an adjunctive approach to pharmacologic, radiation, and ablative therapies in the treatment of solid malignancies. However, limitations in its use as a tumor-specific vector may be present due to preferential colonization within nutrient-rich and hypoxic microenvironments of necrosis. Using a syngeneic murine model of mammary carcinoma, experiments were designed to study the spatiotemporal dynamics of bacterial proliferation within tumor microenvironments. With time, bacteria accumulate in the tumor transition zone, a region of quiescence between viable tumor and necrosis. An increase in tumor apoptosis and a decrease in tumor growth were noted at two days aftera single systemic treatment; this response was abrogated by four days after treatment. Bacterial specificity in colonization was noted towards subcutaneous tumors compared to liver, as well as hepatic metastases compared to normal liver. Using knowledge of observed patterns in bacterial accumulation it was determined that by two days adequate colonization in viable tumor and optimal effect in tumor apoptosis was achieved. Subsequently, strains were electroporated with radiation-inducible prokaryotic-expression plasmids to allow for tumor-specific production of either a green fluorescent protein (control) or murine tumor necrosis factor related apoptosis-inducing ligand (TRAIL). The effect of systemic infection of mice with subcutaneous mammary tumors was examined, comparing the administration of bacterial vectors with or without addition of 2Gy gamma radiation at two days after colonization. The combination of systemic infection with attenuated S. typhimurium and gamma-irradiation conferred a significant increase in tumor doubling time. The expression vector for TRAIL under a radiation-inducible promoter conferred a significant improvement in survival, with flattening of tumor growth curves after induction by radiation. Repeated dosing and irradiation after one week limited tumor growth from baseline, with a significant survival benefit. By capitalizing on the intrinsic motility of bacteria and their preferred microenvironments within tumors in time, the therapeutic utility of targeted therapy using attenuated Salmonella typhimurium as a TRAIL expression vector has been demonstrated in vivo.