Vectors were shuttled into L. monocytogenes by the use of E. coli strain S17 or strain S10 through conjugation (60). In-frame deletions of genes in L. monocytogenes were performed by allelic exchange (61) using suicide plasmid pksv7-oriT as previously described (62). Integrative vector pIMK2 (63) was used for constitutive expression of L. monocytogenes genes. Transductions between L. monocytogenes strains were performed using U153 bacteriophage (64).