Different human tumour types harbour their own unique bacterial communities, according to a new and comprehensive microbiome study described in the journal Science.

It has long been known that bacteria are present in tumours that originate from tissues routinely exposed to microbes, such as in the gastrointestinal tract, but whether they also are found in tumours arising from “sterile” tissues has been less clear.

To investigate, an international team led by Deborah Nejman from Israel’s Weizmann Institute of Science collected more than 1500 samples of tumours and adjacent normal tissue from nine medical centres in four countries.

They included melanoma, bone and brain cancer – all tumour types whose association with bacteria had not previously been explored.

Applying a variety of methods to detect bacterial DNA, RNA and protein, the researchers found that most tumours and their adjacent normal tissues harbour bacteria.

Different tumour types had distinct microbiomes, with breast tumours displaying a particularly rich and diverse community of bacteria, and bacteria within the tumours were present in both cancer cells and immune cells.

There also were “intriguing” associations between specific species of intra-tumour bacteria and factors such as patient smoking status, but Nejman and colleagues stress that further work will be required to determine whether and how intratumor bacteria contribute to tumour development, progression and response to therapy.

Indeed, it’s all still very much a work in progress, with the authors acknowledging in their paper that their data “do not establish whether intra-tumour bacteria play a causal role in the development of cancer or whether their presence simply reflects infections of established tumours”.

Nonetheless, the work raises multiple important questions for future study, Chloe Atreya and Peter Turnbaugh, from the University of California, US, say in a related commentary. For one, is the level of diversity and physiological status of these bacterial cells sufficient to constitute a “microbiota”?

“Achieving a comprehensive understanding of the tumour 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,” they write.