Following three washes in PBS containing 0.05% Tween 20 (PBST), the plates were blocked with 5% skimmed milk, 1% BSA, 1% gelatin or 1% ovalbumin at 37 8C for 2 h, then incubated again with 100 ml swab suspensions at 37 8C for different times (from 0.5 to 2 h).