Prostatitis is an inflammation of the prostate gland. It is sometimes caused by bacterial infection which can either be acute -- typically cured with antibiotics, or chronic -- which is more difficult to treat and occurs more frequently in men with gastrointestinal disturbances, such as irritable bowel syndrome (IBS).
A study among men with chronic prostatitis and IBS found that taking a probiotic along with antibacterial treatment helped reduce the risk of recurrent infection and the progression of inflammation to other glands. Details about this study and probiotic used are found in the "What It Does" section of the Probiotic Supplements Review
There do not appear to be studies evaluating the benefits of probiotics for acute bacterial prostatitis or for chronic non-bacterial prostatitis — which is the most common form of prostatitis.