Question

7. Describe the type of motility of Bacillus cereus under the low-power objective and high-power objective. 8. Design a test where one can manipulate the environment or add a dependent variable causing a change in motility 9. Which of the inoculated media tested positive for motility? How can you tell? 10. Using a microbiology textbook or other resource(s) describe any other bacteria that may test negative for motility

Answer 1

7. Microscopy is the most accurate way to determine bacterial motility and 'Hanging drop method' is a commonly used microscopic technique

In lab, organisms like Bacillus cereus are use to determine motility using Stab technique.

In Stab technique:  

1. Wire with organisms is brought into tube without touching walls of tube
2. Penetrate medium two thirds of its depth
3. Wire is withdrawn from medium and tube. Neck of tube is flamed and plugged

Bacillus cereus was motile because in the tube it showed that the organisms swam away from the line of inoculation into the unioculated surrounding medium, causing the medium to be turbid.

Small amounts of agar and gelatin are added to provide a semi-solid medium which allows macroscopic examination of bacterial motility.Organisms are stabbed into the medium which is then incubated for a 24 hour period. Motile organisms extend from the stab line and produce turbidity or cloudiness throughout the medium. Non-motile organisms grow only along the stab line and leave the surrounding medium clear.

SIM agar may also be used to detect the presence of H₂S production. The SIM medium contains peptones and sodium thiosulfate as substrates, and ferrous ammonium sulfate, Fe(NH₄)SO₄, as the H₂S indicator. Cysteine is a component of the peptones used in SIM medium. Sufficient agar is present to make the medium semisolid. Once H₂S is produced, it combines with the ferrous ammonium sulfate, forming an insoluble, black ferrous sulfide precipitate that can be seen along the line of the stab inoculation. If the organism is also motile, the entire tube may turn black. This black line or tube indicates a positive H₂S reaction; absence of a black precipitate indicates a negative reaction.

E. aerogenes produce pink or red mucoid colonies; P. Vulgaris grow on agar but do not ferment lactose (appears yellow to light pink)
Salmonella- positive, motile. Shigella- negative, nonmotile. Escherichia- negative, motile.

In agar stab method Proteus vulgaris showed a well dispersed growth from the line of inoculation which is evident of motility, presence of black precipitate indicates that H2S is produced and presence of red colouration in the medium after the addition of Kovac’s reagent indicates positive indole test Gram positive bacteria like Klebsiella pneumoniae is non motile and negative for H2S production and indole test

Motility test is also used for the species differentiation of gram positive cocci, Enterococci. Enterococcus faecium and E. faecalis are non-motile, whereas E. gallinarum and E. casseliflavus/E. flavescens generally are motile