Phase variation caused by inversion. (A) Two genotypes (blue and green circles) are consistently and reproducibly prevalent whenever a single bacterium is grown to a population in a phenomenon termed phase variation. The two genotypes are distinguishable by a genomic inversion - a mutation which occurs when a fragment of DNA residing between two inverted repeats (IRs) is detached from the chromosome, and is then reattached in a reverse manner, resulting in a switch between the two strands. The two phenotypes may differ, for example, if a promoter located inside the fragment changes orientation and alters the transcription (gray arrow) of genes outside the inverted segment. (B) Phase variation in the fim operon. A DNA segment (shaded area) containing the fimA promoter can switch between two phases: an ON phase, where the promoter is correctly oriented, and the fim operon is expressed, and an OFF stage, where it is silenced. The OFF state also destabilizes the DNA recombinase fimE, probably by transcribing its antisense.