I visited the one and only microbe museum-, Micropia in Amsterdam in August, following the ESEB meeting in Groningen.  Highly recommend for anyone with an interest in microbes or science in general.  Very interactive.

Fixation and loss of resistance mutations

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.