Ultrastructural analysis of the cell surface of C. diphtheriae strains. (A) Bacteria were fixed on glass slides by drying using compressed air. Atomic force microscopy was carried out under ambient laboratory conditions and operated in tapping mode. Scale bars: 500 nm. (B) AFM images were analyzed in respect to pili number per bacterium. For each strain pili of at least six bacteria were counted; error bars indicate deviations from mean values.