Discarded cancer DNA hides 18 new bacterial species

Somewhere in the archives of cancer research sits a large pile of data that everyone was paid for and nobody looked at twice. Each time a tumour or a blood sample is sequenced, a small share of the reads belong to no human at all. They are filtered out as 'noise' and discarded. Researchers at the University of East Anglia (UEA) decided to look in that pile, and they found 18 new bacterial species in it.

The method is a kind of genetic jigsaw. The team used metagenomic assembly, which rebuilds whole genomes from short fragments of mixed genetic material, and applied it to 2,176 sequencing datasets from the Pan Prostate Cancer Group, a consortium of researchers from nine countries that holds molecular data from more than 2,000 prostate cancers. From those fragments they recovered 249 near-complete bacterial genomes from 194 patients. Twenty-four of them matched no species on record. Most of the organisms came from the mouth, and the most frequently recovered genus was Porphyromonas, which includes the main bacterium behind gum disease. The team has given formal provisional names to 16 of the new organisms, including Candidatus Porphyromonas leptis, Candidatus Treponema oralis and Candidatus Nanosyncoccus prostatocharitus. The word Candidatus marks a bacterium described from its genome alone, before anyone has managed to grow it in a lab.

The context explains why the discovery matters. Oral bacteria are already known to play roles elsewhere in the body, and in 2022 the UEA team reported that men carrying a set of anaerobic bacteria, several from the mouth, were more than twice as likely to see their cancer progress. Bowel cancer research has gone further, treating one oral bacterium as a target. Prostate cancer has lagged, and according to lead researcher Daniel Brewer, the reason is practical: the organisms involved had never been properly characterised. "A biomarker is only useful if you can measure it," he said, "and you cannot measure an organism that has no genome and no test."

For clinicians and researchers, the gain is concrete, though not immediate. First author Abraham Gihawi said the genomes now make it possible to develop tests for these bacteria in clinical samples, to work out how to grow them, and to study their genes. The team has already validated a DNA test that detects one species with precision. Whether any of these bacteria drive aggressive disease, and whether treating them would change the outcome, remains an open question. The study itself is described as early-stage, and the researchers say the sequencing archives of cancer centres worldwide may hold more such evidence.