Characterisation by motility testing: Hanging drop method

/Characterisation by motility testing: Hanging drop method
Characterisation by motility testing: Hanging drop method2018-02-23T12:22:28+00:00

Characterisation by motility testing: Hanging drop method

Sampling of probiotic bacteria from food samples and isolation on specialised enriched media

Introduction

Many bacteria show no motion and are termed nonmotile. However, in an aqueous environment, these same bacteria appear to be moving erratically. This erratic movement is due to Brownian movement. Brownian movement results from the random motion of the water molecules bombarding the bacteria and causing them to move.

True motility (self-propulsion) has been recognized in other bacteria and involves several different mechanisms. Bacteria that possess flagella exhibit flagellar motion. Helical-shaped spirochetes have axial fibrils (modified flagella that wrap around the bacterium) that form axial filaments. These spirochetes move in a corkscrew- and bending-type motion. Other bacteria simply slide over moist surfaces in a form of gliding motion.

The above types of motility or nonmotility can be observed over a long period in a hanging drop slide. Hanging drop slides are also useful in observing the general shape of living bacteria and the arrangement of bacterial cells when they associate together. A ring of Vaseline around the edge of the coverslip keeps the slide from drying out.

Requirements

  1. Bacterial culture
  2. Lens paper and lens cleaner
  3. Immersion oil
  4. Clean depression slides and coverslips
  5. Petroleum jelly (Vaseline)
  6. Inoculating loop
  7. Toothpicks
  8. Bunsen burner

Procedure

  1. With a toothpick, spread a small ring of Vaseline around the concavity of a depression slide. Do not use too much Vaseline.
  2. After thoroughly mixing one of the cultures, use the inoculating loop to aseptically place a small drop of one of the bacterial suspensions in the center of a coverslip.
  3. Lower the depression slide, with the concavity facing down, onto the coverslip so that the drop protrudes into the center of the concavity of the slide. Press gently to form a seal.
  4. Turn the hanging drop slide over and place on the stage of the microscope so that the drop is over the light hole.
  5. Examine the drop by first locating its edge under low power and focusing on the drop. Switch to the high-dry objective. In order to see the bacteria clearly, close the diaphragm as much as possible for increased contrast. Note bacterial shape, size, arrangement, and motility. Be careful to distinguish between motility and Brownian movement.
  6. Discard your coverslips and any contaminated slides in a container with disinfectant solution.
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