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Abstract The members of the infant microbiome are governed by feeding method (breastmilk vs. formula). Regardless of the source of nutrition, a competitive growth advantage can be provided to commensals through prebiotics – either human milk oligosaccharides (HMOs) or plant oligosaccharides that are supplemented into formula. To characterize how prebiotics modulate commensal – pathogen interactions, we have designed and studied a minimal microbiome where a pathogen,Streptococcus agalactiaeengages with a commensal,Streptococcus salivarius. We discovered that whileS. agalactiaesuppresses the growth ofS. salivariusvia increased lactic acid production, galacto‐oligosaccharides (GOS) supplementation reverses the effect. This result has major implications in characterizing how single species survive in the gut, what niche they occupy, and how they engage with other community members. 

Abstract Group BStreptococcus(GBS) is an encapsulated Gram‐positive bacterial pathogen that causes severe perinatal infections. Human milk oligosaccharides (HMOs) are short‐chain sugars that have recently been shown to possess antimicrobial and anti‐biofilm activity against a variety of bacterial pathogens, including GBS. We have expanded these studies to demonstrate that HMOs can inhibit and dismantle biofilm in both invasive and colonizing strains of GBS. A cohort of 30 diverse strains of GBS were analyzed for susceptibility to HMO‐dependent biofilm inhibition or destruction. HMOs were significantly effective at inhibiting biofilm in capsular‐type‐ and sequence‐type‐specific fashion, with significant efficacy in CpsIb, CpsII, CpsIII, CpsV, and CpsVI strains as well as ST‐1, ST‐12, ST‐19, and ST‐23 strains. Interestingly, CpsIa as well as ST‐7 and ST‐17 were not susceptible to the anti‐biofilm activity of HMOs, underscoring the strain‐specific effects of these important antimicrobial molecules against the perinatal pathogenStreptococcus agalactiae.