Bacterial enteropathogens encounter in the intestine a number of microorganims that in various ways can protect the host against the invading microbe. Listeria monocytogenes is a food borne pathogen which invades the intestinal epithelium mainly at the level of the goblet cells. We have identified in Listeria genomes, genes encoding bacteriocins, i.e. toxins that are active against bacteria. The first bacteriocin named LLS which is present in most epidemic strains is poorly expressed in vitro but is highly expressed in vivo. Its presence is critical for oral infection and correlates with a decrease in the amount of Allobaculum and Alloprevotella in the intestine but it is still unknown whether this effect is direct or not. The second bacteriocin is expressed in the majority of clinical strains. Quite strikingly it targets specifically Prevotella copri in both the mouse and the human gut microbiota. Expression of the bacteriocin lowers infection, revealing for the first time an unsuspected role for Prevotella copri in exacerbating intestinal bacterial infection.