FIG 7 .

Summary model for concentration-dependent effects of chloramphenicol on B. subtilis. The competitive culture format for S. venezuelae and B. subtilis suggests a model for the spatial and temporal effects of population growth on production and diffusion of chloramphenicol in the agar medium. (A) Early (~24 h) development of the S. venezuelae strain (light green spots) results in low concentrations (yellow) of chloramphenicol in the medium, sufficient for stimulating sliding motility in the proximal B. subtilis strain (light tan shapes). (B) Continued growth (~48 h) and, presumably, chloramphenicol biosynthesis by the proximal S. venezuelae spot are impeded by the migratory population of B. subtilis. During this time, the more distal spots of S. venezuelae grow to a greater extent and produce higher yields of chloramphenicol. The concentration of chloramphenicol (and possibly other, unidentified metabolites) becomes sufficient (red) to impede growth and progression of the sliding population of B. subtilis, which is therefore prevented from contacting the S. venezuelae population. The unaffected populations of B. subtilis (not mobilized by chloramphenicol exposure) are visible as dark tan spots.