See also Bacterial structure
Bacteria have various appendages for motility (flagella and axial filaments). The primary function of flagella is to confer motility or sel propulsion. Motility is a piee of information used in the lab for identificaiton or diagnosis of pathogens. Flgagella are hard to visualize via light microscopy, but often it is sufficient to know simply wheter a bacteral species is motile. One way to detect motility is to stab a tiny mass of cells into a soft (semisolid) medium in a test tube. Growth spreading rapidly thorugh the entire medium is indicative of motility.
Bacteria can move in response to chemical signals in a process called chemotaxis. The flagellum guides bacteria. There are clusters of receptors located in the cytoplasmic membrane that bind specific molecules coming from the immediate environment. The attachment of sufficient numbers of these molecules transmits signals to the flagellum and sets it into rotary motion. The fuel for the flagellum to turn is a gradient of protons (hydrogen ions) that are generated by the metabolism of the bacterium and that bind to and detach from parts of the flagellar motor within the cytoplasm.
Motility Test: The motility test uses a semisolid medium. Agar is typically 1.5% to 0.4% which is just enough to maintain its form while allowing movement of motile bacteria. A tetrazolium salt (TTc) can be added in the medium. TTC is used as an electron acceptor. When it gets reduced it is red and insoluble. A positive result for motility is indicated whne the red (reduced) TTC radiates outward from a central stabl. A negative result shows red only along the stab line.
Experiment: label tubes having the motility media (with tape) with 1. proteus vulgaris (should show + results), B. cerus (should show + results), S. Aureus ((should show – results) and a control. Stab the tubes 1/2-3/4 way. Incubate at 37C.
Wet Mount Preparation: is made by placing your specimen in a drop of water on a microscope slide and then covering it with a cover glass. Because no stain is used and most cells are transparent viewing is best done with as little illumination as possible. Viewing must be done quickly because of drying of your preparation. Motility is detected by independent darting of the cells. Be careful not to mistake motility with bacteria appearing to herd across the filed which is due to the water receding.
Hanging Drop Preparation: allows longer observation of your specimen because it does not dry out as quickly. A thin ring of petroleum jelly is applied around the well of a depression slide. A drop of water is then placed in the center of the cover glass and living microbes are then transferred into it. A depression microscope slide is next carefully placed over the cover glass in such a way that the drop is received into the depression and is undistrubed. The petroleum jelly will cause the cover glass to stick to the slide. The preparation can now be picked up inverted so the cover glass is on top and placed under a microscope. As with the wet mount viewing is best down with as little illumination as possible. Motility is observed when cells move over greater distances than simply the motility caused by Brownian motion created by colission with water molecules.