In vitro infectivity. Numbers of invaded un-restricted (grey) and oxygen-restricted (white) L. monocytogenes-CFP per well of Caco-2 cells grown to selected densities. Counts were normalized to a concentration of 107 L. monocytogenes per ml in the bacterial cultures prior to infection. Each bar represents an average of three different experiments. Error bars designate standard deviations. OD600 refers to optical density at 600 nm.