Motility
There are a number of methods for determining whether or not bacteria are capable of independent movement or motility. True motile organisms produce flagella (see glossary) and show purposeful movements which may be in different directions to the flow of any fluids in the environment. Some bacteria produce more flagella than others and tend to be more “robustly” motile. Others produce only a single flagellum and while some of these are still robustly motile, others show more limited movements.
Production of flagella and motility is at its peak during log phase of growth. Hence a log phase culture should always be used to assess motility, otherwise moribund or dead organisms may be mistaken for non motile cells.
SIMPLE WET MOUNT
This technique is suitable for robustly motile organisms.
- Grow the organisms to be tested to log phase in liquid culture, usually 2-3 hours is sufficient.
- Place a drop of broth on a slide and add a coverslip.
- Examine with the 40x lens on the microscope. It is preferable to use a phase contrast microscope.
- Observe for darting, twisting or tumbling motions. Small “jiggling” movements are due to random Brownian motion and are not true motility.
HANGING DROP TECHNIQUE
It is essential to use this technique for those organisms which are not as robustly motile. With this technique, the organisms are not pressed between the slide and the coverslip and are more free to move around. It is also advisable to use a phase contrast microscope for this technique.
- Grow the organisms to be tested to log phase in liquid culture, usually 2-3 hours is sufficient.
- Place a coverslip onto a square of blotting paper. Take 2 small pieces of plasticine and roll them into narrow tubes. Place one on opposite edges of the coverslip.
- Place a drop of culture in the centre of the coverslip.
- GENTLY press a microscope slide onto the rolls of plasticine such that the slide adheres.
- QUICKLY turn the slide over such that the coverslip is uppermost and the drop of culture is hanging down from the coverslip but is not touching the slide.
Examine the culture under the 40X lens in the phase contrast microscope.