Invasion of epithelial cells by C. diphtheriae strains. D562 cells were infected with different C. diphtheriae strains (DSM43989 tox+, all others are non-toxigenic), washed, and incubated 2.0 (A), 8.5 (B) and 18.5 (C) hours with 100 μg ml-1 gentamicin. Subsequently, cells were washed, detached with trypsin solution, lysed with Tween 20, and the number of intracellular cfus was determined. Invasion is shown as percentage of the inoculum internalized (means and standard deviations of three independent biological replicates with three samples each (technical replicates). Statistically relevant differences between the strains (based on students TTEST values below 0.05) are indicated by letters above columns.