Abstract: The present invention relates to characterizing changes in mammalian gastrointestinal microbiota associated with antibiotic treatment and various disease conditions (such as obesity, metabolic syndrome, insulin-deficiency or insulin-resistance related disorders, glucose intolerance, diabetes, non-alcoholic fatty liver, abnormal lipid metabolism, short stature, osteoporosis, and other disorders of bone formation and mineralization, etc.) and related diagnostic and therapeutic methods. Therapeutic methods of the invention involve the use of probiotics, prebiotics, or narrow spectrum antibiotics/anti-bacterial agents that are capable of restoring healthy mammalian bacterial gastrointestinal microbiota.