If someone came to you one day and said, "Let's throw some bacteria into a pill and you'll swallow it and then let's watch your brain, liver, heart, erection and mood function miraculously improve!" - you'd tell him he was crazy, right? Yet researchers have found that replenishing certain intestinal bacteria, or probiotics, does all of that and much, much more.
It kind of makes sense when you stop to think that "you are what you eat" and what you eat has to pass through your small intestine and all the bacterial inhabitants that live therein. Colon health may not be glamorous, but with every passing month researchers find how it is one of the most important aspects of your personal well-being and fitness.
Let's look at five counterintuitvie - one might almost say "weird" - ways that probiotics have been found to affect male health:
1. Heart Disease (and Erections). Research is preliminary and somewhat speculative at this point, but it is very likely that probiotics can improve cardiovascular disease and erectile dysfunction in many men. The reason? One study explained that probiotics help the arteries in three key ways: a) increasing and improving t-cell response which relaxes our normally "overzealous" immune system, meaning less white blood cells will end up creating plaque along the endothelium, b) secretion of proprionic acid which signals the liver to produce less cholesterol and c) forcing the liver to use more cholesterol which, in turn, lowers plasma total cholesterol numbers.  All of this will be good for men struggling with erectile strength and ensuing studies will undoubtedly show their benefit.
NOTE: Inflammation is incredibly important to both erectile strength and heart health. Please see my link on How to Lower Inflammation for more information.
2. Anxiety. An animal study found that administration of an antibiotic "cocktail," which kills most intestinal flora, led to an increase in anxiety in the mice. Upon examination, they found that the mice has an increase in BDNF, which is known to increase anxiety.  This may explain part of the reason why researchers have noticed that antibiotic use is often associated with stress reactions, depression and anxiety.
3. Mood. Reseachers studied those that consumed a probiotic containing milk drink in a double-blind placebo-controlled study of seniors.  The study results clearly showed that mood improved in those taking the protbiotics. Again, who would have guessed that probiotics are so good for the brain?
4. Liver. Researchers discovered yet another bizarre connection with gut flora: TNF-alpha. You may recall that TNF-alpha is the bad boy inflammatory cytokine that is a root cause of many other chronic conditions that plague modern urban-dwelling men. Thus researchers have found (in an animal study) that better control of TNF-alpha led to big improvements in Non-Fatty Liver Disease.  Some probiotics have beens shown to improve IL-10, an anti-inflammatory cytokine, and other parameters of inflammation as well.
NOTE: This improvement in TNF-alpha is yet another reason that probiotics may help many men with erectile dysfunction. Please see my link on Juice and Inflammation for more information.
5. Immunity. Several studies have found that various probiotics help immune function. Researchers have found that it likely helps with childhood dermatitis , ulcerative colitis , H.Pylori/ulcers  and on and on. Researcher do not totally understand yet how probiotics works its wonders on the immune system, but it seems to be by positive affecting inflammation and the permeability of the intestine.
So what are probiotics good for? It's not too much of an exagerration to say "everything!" Anything that boosts immunity, brain function and improves the body's inflammatory response is a huge factor in personal health.
1) Gastroenterology, Aug 2011, 141(2):599-609, "The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behavior in Mice"
2) European Journal of Clinical Nutrition, 2007, 61:355 361, "Impact of consuming a milk drink containing a probiotic on mood and cognition"
3) Hepatology, Feb 2003, 37(2):343 350, "Probiotics and antibodies to TNF inhibit inflammatory activity and improve nonalcoholic fatty liver disease"
4) Journal of Allergy and Clinical Immunology, Feb 2003, 111(2):389-395, "Effect of probiotic Lactobacillus strains in children with atopic dermatitis"
5) World J Gastroenterol, 2008 Apr 28; 14(16):2511 2516, "Anti-inflammatory activity of probiotic Bifidobacterium: Enhancement of IL-10 production in peripheral blood mononuclear cells from ulcerative colitis patients and inhibition of IL-8 secretion in HT-29 cells"
6) Alimentary Pharmacology & Therapeutics, Apr 2006, 23(8):1077 1086, April 2006, "Systematic review: are probiotics useful in controlling gastric colonization by Helicobacter pylori?"
7) Journal of Cardiovascular Disease and Research, 2010, 1(4):213-214, "Potential of probiotics in controlling cardiovascular diseases"
8) Infection and Immunity, Sep 2004, 72(9):5308-5314, "Live Lactobacillus reuteri Is Essential for the Inhibitory Effect on Tumor Necrosis Factor Alpha-Induced Interleukin-8 Expression"