Patients in Groups B and C will begin receiving kefir on the same day that their prescribed antibiotic regimen begins. Four ounces of kefir will be administered 3 times a day to patients enrolled on this study. A 4 oz. glass of kefir will be provided with every mealtray, and nursing staff at each of the participating institutions will administer the kefir to the patients. The amount of kefir consumed by the patient will be recorded by the nursing staff and entered in the electronic medical record. Four ounces is a standard serving of yogurt and is well-tolerated by patients. This serving size will help maintain ongoing live probiotic cultures in the individuals receiving kefir.