The aims are to 1) compare two probiotic treatments (multi-strain synbiotic vs. multi-strain probiotic) on bifidobacteria fecal colonization counts at 1, 2, 3, and 4 weeks of life, 34 weeks corrected gestation age (CGA) ; 2) compare infants successfully colonized with probiotic organisms to infants not successfully colonized at 1, 2, 3, and 4 weeks of life, 34 weeks CGA on infant outcomes and on stress biomarker patterns at birth, day of life (DOL) 1, DOL 7; 3) determine long-term safety and outcomes of probiotic treatments at 6, 16, and 24 months CGA.