Plasma concentration of Propionibacterium acnes antibodies and prostate cancer risk: results from an Australian population-based case–control study

BACKGROUND: Recent studies in prostatic tissue suggest that Propionibacterium acnes (P. acnes), a bacterium associated with acne that normally lives on the skin, is the most prevalent bacterium in the prostate and in men with benign prostatic hyperplasia. Its prevalence is higher in samples from patients subsequently diagnosed with prostate cancer. The aim of our study was to test whether circulating levels of P. acnes antibodies are associated with prostate cancer risk and tumour characteristics using plasma samples from a population-based case–control study. METHODS: We measured plasma concentration of P. acnes antibodies for 809 cases and 584 controls using a recently developed ELISA assay. We compared antibody titres between cases and controls using unconditional logistic regression adjusted for batch and variables associated with the study design (i.e., age, year of selection and centre). The primary analysis included P. acnes titres in the model as a dichotomous variable using the median value for controls as the cut-off value. RESULTS: P. acnes antibody titres for both cases and controls ranged from 1 : 16 (i.e., low concentration) to 1 : 65 536 (i.e., high concentration; median value=1 : 1024). The odds ratio for prostate cancer associated with titres at or above the median value was 0.73 (95% CI 0.58–0.91, P=0.005). The association appeared to be particularly strong for advanced prostate cancer (AJCC Stage grouping III–IV) for which the odds ratio was 0.59 (95% CI 0.43–0.81, P=0.001) but there was insufficient evidence that the association differed by tumour stage (p heterogeneity=0.07). CONCLUSION: These results need to be confirmed in prospective studies but they are consistent with the hypothesis that P. acnes has a role in prostate cancer.