Intravenously injected bacteria have the ability to enter and replicate within tumors in mice. Here, we further characterized this dynamic process by investigating the conditions required for tumor colonization by bacteria. We discovered that a broad range of bacteria, including both Gram-negative and Gram-positive strains, colonize a panel of syngeneic and xenogeneic tumors, as well as spontaneous tumors in mice. The colonization process is dependent on the stage of tumor development, but independent of tumor type. The entry and replication of bacteria in tumors was consistently achieved in nude mice when approximately 1 X 10(5) bacteria were injected. The majority of the bacteria were found in the central portion of the tumors, coinciding with the necrotic regions of sectioned tumors. We also found that, although cancer can be characterized as a chronic inflammation, inflammation alone, as seen in cutaneous wounds and in sites artificially induced by Sephadex, was not sufficient to sustain bacterial colonization. Furthermore, we found that "sensitizing" the tumors by oncolytic vaccinia virus colonization prior to bacterial delivery may help to enhance tumor colonization at least twofold by bacteria that were delivered subsequently. Taken together, the data from this and previous studies, lead us to propose the concept that blood-borne bacteria enter tumors to amplify in the central necrotic region; and that, due to impaired immunosurveillance in the tumors as known previously, the clearance of bacteria is inhibited. Bacterial colonization of tumors could be achieved without any specific gene modifications either in bacteria or in the host.