Association of CFSE-stained Bb-HP with human neutrophils. Neutrophils and CFSE-stained or unstained (PBS) Bb-HP were incubated for 5 (A) or 30 (B) min before the reaction was stopped by adding ice-cold PBS and samples were fixed. Flow cytometry was performed using "cell settings" showing only fluorescence associated with neutrophils. Panels (A) and (B) show histograms representative of five to eight different experiments carried out with neutrophils isolated from four to seven different donors. Association of CFSE stained borreliae with the cells is shown as grey histograms and of unstained control bacteria as white histograms. Percentages of fluorescent neutrophils are shown above the histograms as mean ± standard deviation (SD) of five to eight different experiments carried out with cells from four to seven different donors.