Irritable bowel syndrome (IBS) is a common disorder that causes gas, bloating, cramping and diarrhea or constipation.
Experts think the process begins with excessive gas released by microbial digestion of fiber in the small intestine. The large intestines are better sites for fiber digestion but often disruptions cause backup: change in acidity or slow transit times are two. Medications are common disruptors: antacids will change the pH, antidepressants will slow the transit time and antibiotics will mess up the microbial ecosystem.
IBS can also be triggered by inflammation caused by a bout of stomach flu. When inflammation persists, nerves in the tissue become hypersensitive causing pain and cramping.
Probiotics reduce the first problem, overgrowth, through release of chemicals. And inflammation is lessened when probiotics stimulate regulatory T cells. Both animal and human studies are building for this role for probiotics. Effects on each symptom are likely to be species-specific.
Further reading on irritable bowel syndrome and probiotics: