FIG. 4.

IID is not due to a freely diffusible lytic activity. A strain of RHΔ that was transformed with a SAG1-driven β-galactosidase reporter (24; RHΔ-βgal) was used as the Iid⁺ strain and mixed with RHΔ, MBE1.1, MBE3.3, or MBD2.1 in a 1:1 ratio. The extracellular parasites were incubated in 1 μM A23187 in HBSS_c at 37°C for periods of 0 to 60 min, diluted, and plated on HFF monolayers for the development of plaques. After 5 days, the monolayers were fixed and stained with 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside (X-Gal) to distinguish the plaques from each strain. After counting the blue plaques, the monolayer was counterstained with crystal violet to score all the remaining “colorless” plaques. (a) Control showing the expected drop in EOP of the Iid⁺ RHΔ-βgal strain in the four coincubation assays from panel b (scoring for blue plaques). (b) The effect of coincubation with RHΔ-βgal on the IID phenotype of the four strains.