The mechanisms by which probiotic strains enhance the health of the host remain largely uncharacterized. Here we demonstrate that Lactobacillus salivarius UCC118, a recently sequenced and genetically tractable probiotic strain of human origin, produces a bacteriocin in vivo that can significantly protect mice against infection with the invasive foodborne pathogen Listeria monocytogenes. A stable mutant of Lb. salivarius UCC118 that is unable to produce the Abp118 bacteriocin also failed to protect mice against infection with two strains of L. monocytogenes, EGDe and LO28, confirming that bacteriocin production is the primary mediator of protection against this organism. Furthermore, Lb. salivarius UCC118 did not offer any protection when mice were infected with a strain of L. monocytogenes expressing the cognate Abp118 immunity protein AbpIM, confirming that the antimicrobial effect is a result of direct antagonism between Lb. salivarius and the pathogen, mediated by the bacteriocin Abp118.