Several optical imaging techniques have been used to monitor bacterial tropisms for cancer. Most such techniques require genetic engineering of the bacteria to express optical reporter genes. This study investigated a novel tumor-targeting strain of bacteria, Rhodobacter sphaeroides 2.4.1 (R. sphaeroides), which naturally emits near-infrared fluorescence, thereby facilitating the visualization of bacterial tropisms for cancer. To determine the penetration depth of bacterial fluorescence, various numbers of cells (from 108 to 1010 CFU) of R. sphaeroides and two types of Escherichia coli, which stably express green fluorescent protein (GFP) or red fluorescent protein (RFP), were injected s.c. or i.m. into mice. Bacterial tropism for cancer was determined after i.v. injection of R. sphaeroides (108 CFU) into mice implanted s.c. with eight types of tumors. The intensity of the fluorescence signal in deep tissue (muscle) from R. sphaeroides was much stronger than from E. coli-expressing GFP or RFP. The near-infrared fluorescence signal from R. sphaeroides was visualized clearly in all types of human or murine tumors via accumulation of bacteria. Analyses of C-reactive protein and procalcitonin concentrations and body weights indicated that i.v. injection of R. sphaeroides does not induce serious systemic immune reactions. This study suggests that R. sphaeroides could be used as a tumor-targeting microorganism for the selective delivery of drugs to tumor tissues without eliciting a systemic immune reaction and for visualizing tumors.