Bacterial strains used in this study are wild type Listeria monocytogenes EGDe serotype 1/2a (Lm) [40] , wild type L. innocua strain (serotype 6a NCTC 11288) [41] transformed with either the recently characterized gram+ve/gram-ve shuttle pUvBBAC+vgc1 vector and referred to as (L.inn::vgc strain) or the pUvBBAC vector without the inserted vgc and referred to as L.inn [18] Bacteria were grown in brain-heart infusion (BHI) (Difco, Augsburg; Germany) broth in presence or absence of 5 mg/ml erythromycin. For each experiment, erythromycin was used as a selective antibiotic for growth of L.inn::vgc and the wild type L.inn harbouring the pUvBBAC vector. Wild type L. monocytogenes was grown in absence of erythromycin. In all experiments, fresh cultures of bacteria, prepared from an overnight culture, were used. Briefly, bacteria were grown in Brain Heart Infusion (BHI) at 37uC, harvested in the exponential growth phase and washed twice with PBS. The pellet was resuspended in PBS and the bacterial concentration was calibrated by optical absorption. Further dilutions were prepared in PBS to obtain required numbers of bacteria for infection.