Chronic constipation may find relief with probiotics. Microbiota is different in a healthy bowel than that of a reluctant system. One way may be that probiotics lower the pH of the colon due to production of short-chain fatty acids (butyric acid, propionic acid, and lactic acid). A lower pH enhances peristalsis in the colon.
Both studies with the elderly and children resulted in shortened transit time and increased frequency of defecation. Strains of Bifidobacterium and Lactobacillus proved to be effective with no harmful side effects.
Further reading on constipation and probiotics: