Stratification according to participating institutions will be done in order to control for center-related differences. Stratification by birth weight will be performed in 2 groups: < 1500 grams and 1501 to 2000 grams. When eligibility criteria are met, infants will be randomly assigned to treatment with probiotics or placebo by using a computerized stratified balanced block randomization design. Assignment to treatment will be accomplished using sealed, sequentially numbered, opaque envelopes, color-coded for strata, available in each NICU pharmacy.