An estimated 100 trillion microorganisms inhabit every normal, healthy bowel, where they keep pathogens (harmful microorganisms) in check, aid digestion and nutrient absorption, and contribute to immune function. If these gut-dwelling bacteria become depleted (usually because of disease, stress, poor diet, or medications like antibiotics) health problems can result. According to Harvard Women's Health Watch, probiotic therapy has been best studied for the treatment of diarrhea. It may also help people with Crohn's disease, irritable bowel syndrome, and other gastrointestinal problems.
Source
www.health.harvard.edu
I am on the SCD diet and have read that depending on the diet you are on, certain probiotics should be avoided. For Example: VSL#3 is said to cause flares for people on the SCD diet.
Single-strain acidophilus probiotic containing 3 billion CFUs per capsule
Kirkman’s Lactobacillus Acidophilus (L. acidophilus) features only that one probiotic strain and is free of disaccharides. This species is recognized as one of the primary friendly lactic acid bacteria residing in a healthy gastrointestinal tract. Research over the past decade has shown that L. acidophilus provides a number of important health benefits including:
• adheres to the GI mucosa and colonizes the intestinal tract
• supports the body's natural defenses (immunity)
• helps control diarrhea or constipation
• helps alleviate occasional diarrhea or constipation
Source
http://kirkmanlabs.com/ProductKirkman/182/1/LactobacillusAcidophilus-Hypoallergenic/
More GREAT Info on Lactobacillus probiotic
http://www.nlm.nih.gov/medlineplus/druginfo/natural/790.html