FIG. 6.

LLO induction is required to mediate cell-to-cell spread in L2 fibroblasts. (A) iLLO (DP-L3885) 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 infected cell monolayers with PBS, a medium-agarose overlay containing gentamicin was added. Intracellular growth and cell-to-cell spread of bacteria were visualized after 96 h by the formation of clearing zones (plaques) within the L2 monolayers. +/− indicates, respectively, the presence or absence of 1 mM IPTG during the initial 1-h infection period or in the agarose overlay. (B) Magnification (×3) of iLLO plate 2 in panel A. Arrows point to representative pinpoint plaques present within the L2 monolayer.