Listeria monocytogenes strain LO28 serotype 1/2c [65] (kindly provided by Thomas Decker, Vienna, Austria) and EGDe serotype 1/2a [66] was grown in Brain Heart Infusion (BHI) infusion or on BHI-plates (Difco, Detroit, MI) at 37uC overnight. The next day, suspensions were diluted and grown until reaching log-phase. Bacteria were then centrifuged, washed several times and resuspended in sterile PBS. Concentrations of bacteria were determined by measurement at OD 600 and confirmed by plating serial dilutions on appropriate agar plates.