In the inflammatory response, transcriptional factors such as nuclear factor-kappaB are activated upon binding of pattern-recognition receptors, proinflammatory cytokine receptors, and antigen receptors. Although NF-kB does not fit the classical definition of an oncogene, it is a powerful activator of the malignant state and regulates the expression of target genes important for cell proliferation, survival, angiogenesis, and tissue repair. Exposure to environmental factors, such as infectious agents, dietary carcinogens, and hormonal imbalances, is thought to lead to injury of the prostate and the development of chronic inflammation. Recent reports showed that Propionibacterium acnes is frequently detected in prostate tissue from patients with prostatitis and prostate cancer, and that the bacterium is associated with acute and chronic prostatic inflammation and might have a role in prostate carcinogenesis. To further investigate the etiologic association between P. acnes and inflammation or carcinogenesis, not only the bacterium but also histologic features of the prostate tissue need to be analyzed in identical histologic sections. The aim of the present study was to locate P. acnes in prostate tissue under light microscopy by enzyme immunohistochemistry. For this purpose, we developed a novel anti-P. acnes monoclonal antibody that reacts with the bacteria in formalin-fixed and paraffin-embedded prostate tissue sections. To evaluate the pathogenic role of this indigenous bacterium in the development of prostate cancer, we examined radical prostatectomy samples obtained from patients with or without prostate cancer by immunohistochemistry with the novel antibody to P. acnes and an antibody to NF-kB, which was used to determine a possible correlation between P. acnes infection and nuclear NF-kB expression in prostate WY 14643 inquirer glands. Furthermore, we analyzed whether P. acnes infection status was associated with prostate cancer risk. Enzyme immunohistochemistry with a novel P.acnes-specific monoclonal antibody enabled us to visualize prostatic P.acnes under a light microscope for histopathologic analysis. In the present study, cocktail immunostaining with both the PAL antibody and anti-NF-kB antibody, followed by morphometric analysis of positive signals on virtual slides, revealed a possible correlation between intraepithelial P. acnes infection and NF-kB activation in prostate glands. A diagnosis of prostatic P. acnes infection needs to be supported by histologic detection of the bacterium in tissue sections, because this indigenous bacterium may cause contamination and tissue invasiveness cannot be evaluated when traditional culture and polymerase chain reaction-based methods are applied. In previous reports, prostatic P. acnes was visualized by fluorescence immunohistochemistry or fluorescence in situ hybridization methods, although a precise histopathologic examination of the prostate lesions cannot be achieved by these methods. In the present study, we used the enzyme immunohistochemistry with the PAL antibody, which reacts with P. acnes with high specificity on routine histologic sections of the formalin-fixed paraffin-embedded prostate tissues.