Figure 1. Photographs and optical microscopy views of the wells showing plaques formed by Chlamydia trachomatis F/IC-Cal3 (A, C, E) and no plaque formed by clinical F persistent strain (B, D, F). A) Single well showing 2 distinct plaques (indicated by arrows). B) Well showing no plaque morphology. C) and D) Optical microscopy image showing plaque areas with little or no neutral red staining (arrows) surrounded by viable cells stained red (magnification ×100). Higher magnification (×400) showed numerous cells that had been infected by reference strain F/IC-Cal3 (E) and the clinical persistent F strain (F).