Scientists say they have identified urine bacteria which are linked to aggressive prostate cancer.
The discovery might provide new ways to spot and even prevent these dangerous tumours, experts hope.
It's too soon to say if the bacteria might cause the cancer, rather than just be a helpful marker.
The University of East Anglia team which found the link plan more work to see if clearing the infection might prevent bad tumours.
Bacterial infection is known to play a part in the development of other cancers - a bug called H. pylori can trigger stomach cancer, for example, and a course of antibiotics can get rid of this risk.
Prostate cancer is not always life-threatening - some tumours grow too slowly to ever cause any problems, and can safely be left alone and simply monitored instead.
The challenge is diagnosing and quickly treating men who have the aggressive, fast-growing ones that harm, while sparing others from unnecessary treatment.
Currently available tests, such as the PSA blood check and a biopsy, are not always able to predict which cancers will be harmful.
For this latest work, published in the journal European Urology Oncology, the researchers studied more than 600 patients with and without prostate cancer, to assess how useful the urine bacterial test was.
They identified five types of bacteria which were common in urine and tissue samples from men whose cancers ultimately went on to be aggressive.
All were types of bacteria that can grow without oxygen. Some were brand new types, never found until now.
Two of the new bacteria species found by the team have been named after two of the study's funders - Porphyromonas bobii, after the The Bob Champion Cancer Trust and Varibaculum prostatecancerukia, after Prostate Cancer UK.
Dr Rachel Hurst, one of the research team, said: "Among the things we don't yet know is how people pick up these bacteria, whether they are causing the cancer, or whether a poor immune response permits the growth of the bacteria.
"But we hope that our findings and future work could lead to new treatment options, that could slow or prevent aggressive prostate cancer from developing. Our work could also lay the foundations for new tests that use bacteria to predict the most effective treatment for each man's cancer."
Her colleague Prof Colin Cooper, who co-led the research, told the BBC he was very confident that the findings were real - they had followed rigorous measures to make sure there was no chance of contamination as they carried out the lab work.
He said it was possible that some of these bacteria make hormones that drive the development of aggressive tumours.
Dr Sam Godfrey from Cancer Research UK, said: "Nearly four in 10 cancers in the UK are linked to known risk factors such as smoking and obesity. But there are other cancer-causing risk factors, such as bacteria, that we are only just beginning to identify.
"More studies are needed to establish how these bacteria are involved in prostate cancer growth, but this research could help lead to new screening and prevention tools that would help reduce the impact of these cancers on society."