Quiz #2

The smear must be fixed

to preserve bacterial cells with minimal shrinkage or distortion when stained.

The Nigrosin or Congo red can be used to detect____

capsule

In the Gram stain, after the third step (treatment with the ethanol), a Gram-positive bacterium would be and a Gram-negative bacterium would be —

purple/colorless

You tried to stain endospores by treating with malachite green and then safranin, but you forgot to steam the slide. What result do you expect?

The endospore would not be green the slide would show mostly red vegetative cells ( false impression that endospores are absent)

When you performed a Gram stain of 48 hr culture of Bacillus subtilis, you found that most vegetative cells were mostly pink with a few scattered purple cells. What can you conclude from this observation?

The bacteria are still Gram+ (purple), but since the culture is old it get weaker making most of the cells to stain pink

Mycobacterium tuberculosis must be stained by acid-fast staining method because it has a waxy lipid called in its cell wall.

Mycolic acid

Explain how you dilute 1 mg/ml Methylene blue solution by a 1:20 dilution in a 10 ml final volume. Also, what is the final concentration of Methylene blue after the dilution?

stoc= fina v/ dilution (0.5)
dilvent: final v - stock (9.5)
final conce: starting conc/: (0.05)

Unambiguously sketch a few cells of the following bacteria as observed under oil immersion microscope following a Gran stain to indicate morphology and arrangement. Hint: One bacterium is Gram-positive and the other is Gram-negative.


Staph Au
Ecoli

Staph Au: Purple, circle, Gram +
Ecoli: Pink, rod, gram -