FIG. 8.

IPTG dose-dependent plaque formation in L2 fibroblasts. iLLO (DH-L616) or wild-type (10403S) bacteria were grown in BHI broth without IPTG and added to monolayers of mouse L2 fibroblasts, which were incubated for 1 h. Following washing of the monolayers with PBS, a medium-agarose overlay containing IPTG and gentamicin was added to the monolayers. The concentration of IPTG present during both the initial 1-h infection period and in the agarose overlay is indicated. Cell-to-cell spread of bacteria was visualized after 96 h by the formation of plaques within the monolayers. The average diameter of 9 or 10 plaques per sample was determined and is given as a percentage of that observed for wild-type infection.