Figure 1:

S. flexneri streaking for isolation on Congo red agar. As shown on the left, a small amount of the frozen stock or stab is spread on a small area of the plate (1). A sterile loop is used to streak the remaining quadrants of the plate (2, 3, and 4), as described in Basic Protocol 1. After overnight incubation at 37°C, wild‐type S. flexneri forms translucent colonies that bind the dye (Crb⁺) and appear red, as shown on the right. Cells that have lost or have deletions in the virulence plasmid produce larger, more opaque, white colonies (Crb⁻).