Newborns are highly susceptible to orally acquired bacterial infections and this is generally attributed to immaturity of their immune system. This study shows that another contributing factor is the neonatal microbiota, which is less effective in mediating colonization resistance. Kim et al. colonized adult germ-free mice with caecal contents from neonatal or adult mice; unlike adult microbiota, neonatal microbiota was unable to prevent colonization of the gut by the bacterial pathogens Salmonella enterica serovar Typhimurium and Citrobacter rodentium. Around 50% of mice colonized with neonatal microbiota died following S. Typhimurium infection, whereas all mice colonized with adult microbiota survived. The absence of Clostridia species from the neonatal microbiota was shown to account for the lack of colonization resistance and administration of Clostridia to neonatal mice protected them against lethal S. Typhimurium infection. The authors showed that the neonatal microbiota supports the acquisition of Clostridia species before weaning, an event that is crucial for subsequent protection against enteric infections.