Sparse initial entrapment of systemically injected Salmonella typhimurium leads to heterogeneous accumulation within tumors.

Blood-borne therapeutics, which rely on diffusion and convection for delivery, often do not accumulate in effective concentrations distant from vasculature and are therefore unable to eradicate all cells within a tumor. Motile bacteria have the potential to overcome the diffusion and pressure gradients that prevent passive materials from penetrating into poorly perfused regions of tumors. Here, we test several proposed mechanisms of Salmonella typhimurium accumulation in tumors, including: (a) entrapment in the chaotic vasculature of tumors; (b) attraction to specific tumor microenvironments; and (c) preferential replication within specific microenvironments. After systemic injection of S. typhimurium into tumor-bearing mice, we used intravital microscopy and histological techniques to quantify their interaction with tumor vasculature. Immediately after injection, few S. typhimurium (<4 in 10,000) adhered to tumor vasculature; most remained passively suspended in the blood. Despite this low initial adhesion, approximately 10,000-fold more S. typhimurium accumulated in tumors than any other organ 1 week after the injection, thus demonstrating their specificity. However, within the tumors, we found that most bacteria were located in necrotic tissue as large colonies far (750 micro m) from functional vasculature. Together, these results suggest that S. typhimurium has limited ability to adhere to tumor vasculature and migrate within tumors and only survives in tissue that becomes necrotic. Although S. typhimurium is a promising delivery vehicle because of its tumor specificity, increasing its intra-tumoral motility should improve its therapeutic effectiveness.