Viability of Bb-HP, Ba and Bg after staining with CFSE. Borreliae treated according to the staining protocol, with the exception of using PBS instead of CFSE, served as negative control. After the staining procedure, 3.2 μM propidium iodide (PI), staining the DNA of dead bacteria red, was added. Heat-killed and PI stained Bb-HP of a representative experiment are also shown to demonstrate the proper functioning of the PI-staining. The percent of PI-negative bacteria was determined by flow cytometry. Values are expressed as mean ± standard deviation from three to four different experiments. p ≥ 0.1 was obtained for each CFSE labelled strain versus corresponding control.