Polar organelle

A polar organelle is a structure at a specialized region of the bacterial polar membrane that is associated with the flagellar apparatus. This flagellum-associated structure can easily be distinguished from the other membrane regions in ultra-thin sections of embedded bacteria by electron microscopy when the cell membrane is orientated perpendicular to the viewing direction. There, the membrane appears slightly thickened with a finely frilled layer facing the inside of the cell. It is also possible to isolate these polar organelles from the bacterial cells and study them in face view in negatively stained preparations.

The polar organelle bears a fine array of attached particles in hexagonal close packing and these have been shown to possess ATPase activity. The polar organelle is found in close juxtaposition to the points of insertion of the bacterial flagella into the plasma membrane, especially where multiple flagella bases are grouped in a region of the cell membrane. It may thus be inferred that the polar organelle could be of importance in the supply and transfer of energy to the bidirectional molecular rotational motor situated at the base of each individual bacterial flagellum (see also electrochemical gradient).

Polarity can be innate within bacteria, even without the presence of a defined polar organelle. Even bacteria that exhibit symmetrical morphology can have polar characteristics due to charged regions within their plasma membrane. Polar regions of bacteria are often systemic and composed of inclusion bodies that can accumulate at charged poles within the bacteria. The fundamental polarization of bacteria has potential to be developed and manipulated to position different polar organelles and protein complexes. An example of this can be found in Escherichia coli, where fluorescence microscopy has been used to observe the polar clustering of chemoreceptors. This self-assembly process illuminated fluorescence-tagged chemoreceptors accumulating significantly at the distinct poles of the E. coli bacteria. This natural assembly does not utilize an anchor or other method to regulate clustering at the poles. The development of localized polar regions in bacteria can arise naturally without the function or the polar organelle.