Strains and plasmids used in this study.

DPC: Dairy Products Research Centre, Moorepark; UCC: University College Cork; NCDO: National Collection of Dairy Organisms; DSMZ: German Collection of Microorganisms and Cell Cultures; ATCC: American Type Culture Collection; NCIMB: National Collection of Industrial, Food and Marine Bacteria; BSAC: British Society for Antimicrobial Chemotherapy; ILSI: International Life Science Institute (MartinWiedmann), ARS: Agricultural Research Service, U.S. Department of Agriculture (Todd Ward).