We propose that this property gives UCC2003 a competitive advantage in iron-limited environments such as the gastrointestinal tract. The importance of iron acquisition mediated nutritional immunity in gut pathogen protection was further demonstrated by the fact that two additionally constructed mutants harboring insertions in either of two presumed ironuptake genes proved to have a decreased ability to confer protection against Salmonella infection in the C. elegans model. In addition and as expected, these mutants were more susceptible to the iron chelators as compared to the parent strain B. breve UCC2003. It has previously been shown that LuxS affects genes involved in iron metabolism in Porphyromonas gingivalis, Vibrio vulnificus, Mannheimia haemolytica and Actinobacillus pleuropneumoniae, while it was also demonstrated that iron availability increases the pathogenic potential of several gastrointestinal pathogens including S. Typhimurium, Citrobacter freundii, E. coli and Listeria monocytogenes. Our results are consistent with the notion that bifidobacteria confer gut pathogen protection by nutritional immunity. This in turn suggests that LuxS/AI-2 can be versatile in various bacterial species and conditions. Since the colonization capacity of a probiotic bacterium is considered to be a prerequisite to exert its beneficial effects, the observation that AI-2-expressing B. breve UCC2003 outcompetes an isogenic derivative lacking this capacity contributes to the elucidation of molecular players and mechanisms of colonization requirements and probiotic effects. Indeed, one application where administration of bifidobacterial strains may positively influence health is in the prevention of necrotizing enterocolitis in premature infants. The precise causative agent of NEC is unknown; however, the preterm infant microbiota has been found to be dominated by pathogenic genera with Proteobacteria and Enterobacteriaceae dominating rather than characteristic species belonging to Bacteroidetes, Clostridium and Bifidobacterium. The dominance of potentially pathogenic bacteria may increase the risk of infection in this vulnerable group. Neonatal nurseries in Finland, Italy and Japan have been routinely and successfully using probiotics as prophylaxis against NEC for over a decade, without ever reporting any adverse effects. Despite the safe use of practice and numerous randomized clinical trials that indicate that probiotics can reduce the incidence of NEC by at least 30%, clinical guidelines by the American Society for Parenteral and Enteral Nutrition express the view that there is currently insufficient data to recommend the use of probiotics in infants at risk of NEC. Integral to the resistance to adopt probiotics as a prophylaxis against NEC in premature infants is the lack of knowledge on the mechanism of action.