New review below .
"This review discusses the biology and behavior of Propionibacterium acnes (P. acnes), a dominant bacterium species of the skin biogeography thought to be associated with transmission, recurrence and severity of disease. More specifically, we discuss the ability of P. acnes to invade and persist in epithelial cells and circulating macrophages to subsequently induce bouts of sarcoidosis, low-grade inflammation and metastatic cell growth in the prostate gland. Finally, we discuss the possibility of P. acnes infiltrating the brain parenchyma to indirectly contribute to pathogenic processes in neurodegenerative disorders such as those observed in Parkinson's disease (PD)."
Some PCa studies have a different perspective. e.g. a 2005 British study :
"Androgen level or androgen activity is implicated in several health outcomes, but its independent role remains controversial. This study investigated the association between history of acne in young adulthood, a marker of hormone activity, and cause-specific mortality in the Glasgow Alumni Cohort Study. Male students who attended Glasgow University between 1948 and 1968 and participated in voluntary health checks reported history of acne (n = 11,232)."
"Students who reported a history of acne had a lower risk of all-cause (hazard ratio = 0.89 ...) and coronary heart disease (hazard ratio = 0.67 ...) mortality but had some evidence of a higher risk of prostate cancer mortality (hazard ratio = 1.67 ...). This study shows that androgen activity during adolescence may protect against coronary heart disease but confer a higher risk of prostate cancer mortality."
Why would P. acnes be a particular problem? Exposure is short compared to a lifetime of sexual activity & the possibility of significant STDs. Interestingly, while one might expect the number of sexual partners to affect PCa risk, marriage turns out to be an important risk factor. Being married for a long time increases risk. Implies that having a lot of unprotected sex is risky, even when neither partner has had an STD. The innocent bacteria that pass between partners may not be so innocent.
Cohen (2005 - Australia)  looked for bacteria in "Prostatic tissue from 34 consecutive patients with prostate cancer ..."
"The predominant microorganism detected was Propionibacterium acnes, found in 35% of prostate samples. A significantly higher degree of prostatic inflammation was observed in cases culture positive for P. acnes ..."
So the inflammatory response might be the culprit. Subclinical inflammation goes unnoticed & untreated, but markers of inflammation correlate with mortality risk.
Platz had a great idea - acne is a common teen nuisance, but it can be severe, requiring long-term use of tetracycline:
"To indirectly explore associations between P. acnes and prostate cancer, we investigated severe acne, as measured by tetracycline use for four or more years, in relation to incident prostate cancer in the Health Professionals Follow-up Study."
"Men who used tetracycline for four or more years had a significantly higher risk of prostate cancer (16 cases, 1,569 person-years) than men who did not use tetracycline (2,071 cases, 304,822 person-years, multivariable-adjusted RR=1.70"
Small number of cases, but interesting.