Question

1. how can you distingish between Brownian motion and true motility

2. although you can see motility in bacteria, why is it difficult to see the flagella that cause the motion?

3. how does the indicator in motility medium function to demonstrate motility

4. what is the apperance of a positive motility test using motility medium S

Answer 1

Answer 1. In Brownian motility, the movement seen is due to the molecules of the solvent medium bombarding with the organism's surface. This causes jerky or irregular movements. Brownian motion is shown by dead cells too. In case of true motility the bacterium moves with the help of special organelles such as flagella, or by the production of slime by the organism (gliding motility). In this case the bacteria moves purposefully towards a substance which can be food, light or oxygen. True motility is shown by living organisms alone.

Answer 2. Flagella are thin (20 µm) and long with some having a length 2-3 times (about 10 µm) the length of the cell. This small size of bacterial flagella cannot be seen under a light microscope. And hence, although we can see the bacterial cell movement under the light microscope, the flagella stays invisible.