Different human tumor types each harbor their own unique bacterial communities, researchers report in a new study that profiled the microbiomes of more than 1,500 individual tumors across seven types of human cancer - the most comprehensive tumor microbiome study to date. It has long been known that bacteria are present in tumors that originate from tissues routinely exposed to microbes (e.g., the gastrointestinal tract) but whether they are present in tumors arising from "sterile" tissues has been less clear. Deborah Nejman and colleagues collected over 1,500 tumor samples and samples of adjacent normal tissue from nine medical centers in four countries. The samples included melanoma, bone and brain cancer, tumor types whose association with bacteria had not previously been explored. Applying a variety of methods to detect bacterial DNA, RNA and protein, and taking rigorous measures to exclude contamination, they found that most tumors and their adjacent normal tissues harbor bacteria. Different tumor types had distinct microbiomes, with breast tumors displaying a particularly rich and diverse community of bacteria. What's more, the authors found that bacteria within the tumors were present in both cancer cells and immune cells. While there were intriguing associations between specific species of intra-tumor bacteria and factors such as patient smoking status, further work will be required to determine whether and how intratumor bacteria contribute to tumor development, progression, and response to therapy. "Achieving a comprehensive understanding of the tumor microenvironment is a daunting yet critical step toward an organism-wide mechanistic model of cancer progression and, if successful, may unlock the next wave of precision cancer diagnostics and therapeutics," write Chloe Atreya and Peter Turnbaugh in a related Perspective.