Multi-center, double-blind, randomized, placebo-controlled trial Primary outcome: Death or nosocomial sepsis Secondary outcomes: NEC, prevalence of Gram-negative pathogens, duration of hospitalization, and frequency of outpatient treatment and re-admission for infectious causes at 6 months post-discharge. Post-hoc analysis will evaluate differences in RCT outcomes in two key subgroups: breast-fed and formula-fed infants and vaginal delivery versus cesarean section, in both probiotic exposed and unexposed infants. In addition, we will look at death and nosocomial sepsis as separate outcomes to ensure there is no differential effect of treatment in either outcome. Location: 11 NICUs in Colombia.