The clinicians who provided care to the infants are blinded to the grouping. Investigators were not involved in the care of these infants. Study group is fed with infloran (Lactobacillus acidophilus and Bifidobacterium bifidus, Swiss Serum and Vaccine Institute Berne, Switzerland) 125mg/kg/dose (CFU 109) twice daily with breast milk for 6 weeks; the control group is fed with breast milk alone. Infloran was stored in refrigerator at a tempreture between 2 -80 C and mixed with breast milk before feeding. Breast milk was either from infant's own mother's milk or from breast milk bank. Both breast milk of the study and control group are prepared by breast milk team who were not involved in the care of the infant and follow the order from a sealed envelop. Feeding is started when the infant*s vital signs were stable, have active bowel movement, without abdominal distension, without bile or blood from the nasogastric tube, and did not have an umbilical artery or umbilical venous catheter in place for at least 24 hours.