Madine-darby canine kidney (MDCK) and African green monkey kidney (Vero) cells (cell bank of the Friedrich-Loeffler-Institute, Federal Research Institute for Animal Health, Greifswald-Insel Riems, Germany) were maintained in Eagle's minimal essential medium (MEM) supplemented with 5% FCS (GIBCO, Paisley, UK). The exponentially growing cells were harvested and seeded at a cell density of 60 000/well in a 96 well microtiter plate (8 mm diameter, Falcon Plastic, NJ) and incubated for 24 h at 37 C with 5% carbondioxide in a 90% humidified chamber so as to form confluent monolayers.