The top plates contain genamicin, and the bottom two rifampicin. The bottom plates are colonies of Pseudomonas aeruginosa rpoB mutants +P518 (left), and S536F (right). The presence of a colony on the top but not the bottom represents the fixation of a rifamicin-sensitive invader (GFP-PA01) introduced into the rifampicin-resistant population. Note the diversity of colony morphology, suggesting divergence from the ancestral genotype has occurred in all three genetic backgrounds.
Danna Gifford 