Recognition of human, rodent, and pig CD55 as receptor by Dr and F1845 adhesins. CHO cell lines expressing each CD55 were plated to confluency. Cell monolayers were inoculated with 108 bacteria/ml (E. coli strain AAEC185 [control] or recombinant strain AAEC185 expressing Dr [P1-Dr] or F1845 [P1-F1845]). After 3 h at 37°C monolayers were processed to determine CFU levels/milliliter. Indexes of cell-associated bacteria (CFU/milliliter) per cell line were calculated on the basis of the higher cell association level obtained for F1845-positive strain (P1-F1845) in CHO-human CD55 (vector-CHO, 1.58 × 104 ± 0.68 × 104 CFU/ml; human CD55-CHO, 1.85 × 106 ± 0.55 × 106 CFU/ml). Data shown represent mean results ± standard deviations of 10 experiments for each cell line. Similar results were obtained with human wild-type IH11128 and C1845 bacteria expressing Dr hemagglutinin and F1845 adhesin, respectively (data not shown).