6.12 Terminal Organelle


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    Terminal Organelle

    Another, familiar, way to get across a surface is to crawl. For this to work, though, you need to be able to change the conformation of your body. This is possible if your cell lacks a cell wall or surface layer, like this Mycoplasma pneumoniae. Like the Mycoplasma genitalium you saw in Chapter 2.1, these cells are intracellular pathogens, so they do not need to buttress their membrane against differences in osmolarity. As a result, they are soft and flexible. This may allow them to use a leg-like internal structure called a terminal organelle to crawl, or “glide,” across a surface. The exact mechanism is still unclear, but one possibility is that a hinge-like conformational change in the terminal organelle extends and contracts the back of the cell with respect to the front, similar to the movement of an inchworm (but less exaggerated). Combined with adhesion proteins on the cell surface, this might propel the cell forward. (You can watch a video of Mycoplasma mobile gliding on Howard Berg’s website.)

    The skeleton-like terminal organelle gives these cells their characteristic flask shape. In combination with their minimal cell envelope, it can also give rise to an unusual method of cell division. In species (or genetically engineered strains) that lack the division protein FtsZ, Mycoplasma cells still manage to divide. They replicate their terminal organelle normally, as you can see this cell has done, and then the two copies simply walk away from each other, stretching the mother cell between them until the membrane pinches off to produce two daughters.

    Keep in mind that these are simply some of the ways we know bacteria and archaea get around, and we continue to discover new ones.