How bacteria maintain location and number of flagella?

Bacteria differ in number and location of their flagella that appear in regular patterns at the cell surface (flagellation pattern). Despite the plethora of bacterial species, only a handful of these patterns exist. The correct flagellation pattern is a prerequisite for motility, but also relates to biofilm formation and the pathogenicity of disease-causing flagellated bacteria. However, the mechanisms that maintain location and number of flagella are far from being understood. Here, we review our knowledge on mechanisms that enable bacteria to maintain their appropriate flagellation pattern. While some peritrichous flagellation patterns might occur by rather simple stochastic processes, other bacterial species appear to rely on landmark systems to define the designated flagellar position. Such landmarks are the Tip system of Caulobacter crescentus or the signal recognition particle (SRP)-GTPase FlhF and the MinD/ParA-type ATPase FlhG (synonyms: FleN, YlxH and MinD2). The latter two proteins constitute a regulatory circuit essential for diverse flagellation patterns in many Gram-positive and negative species. The interactome of FlhF/G (e.g. C-ring proteins FliM, FliN, FliY or the transcriptional regulator FleQ/FlrA) seems evolutionary adapted to meet the specific needs for a respective pattern. This variability highlights the importance of the correct flagellation pattern for motile species.