Question 1

What is the difference between Gram-positive and Gram-negative bacteria?

Accepted Answer

Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet stain, appearing dark purple. Gram-negative bacteria have a thinner peptidoglycan layer and higher lipid content, so they lose the purple stain during decolorization and are counterstained red by safranin, which binds to their lipopolysaccharide layer.

Question 2

Why is heat-fixing used in Gram staining but not in capsule staining?

Accepted Answer

Heat-fixing adheres bacterial cells to the slide and makes them more readily accepting of stains in Gram staining. However, heat-fixing is avoided in capsule staining because it can disrupt or dehydrate the capsule, causing false negatives, and can shrink cells, creating a clearing that mimics a capsule and causes false positives.

Question 3

How does capsule staining reveal bacterial capsules?

Accepted Answer

Capsule staining uses negative staining, applying an acidic colorant like Congo red to stain the background and bacterial cells, while leaving capsules unstained. The capsule appears as a clear halo around the purple-stained bacterial cells against the dark background, making it visible under light microscopy.

Question 4

What are endospores and why are they difficult to stain?

Accepted Answer

Endospores are dormant, tough, non-reproductive structures that bacteria produce under adverse conditions like extreme temperatures or dehydration to ensure survival. They are difficult to stain with standard techniques because they are impermeable to most dyes, requiring specialized methods like the Schaeffer-Fulton technique with malachite green and heat.

Question 5

What colors result from endospore staining and what do they indicate?

Accepted Answer

In endospore staining, vegetative cells appear pinkish-red after safranin counterstaining, while endospores retain the malachite green stain and appear bluish-green. These distinct colors allow differentiation between spore-forming and non-spore-forming bacteria and confirm the presence of spores in a sample that could lead to bacterial contamination.

Question 6

How can Gram staining be used to identify bacterial morphology and arrangement?

Accepted Answer

After Gram staining, bacterial cells can be observed under light microscopy to determine their shape and arrangement. Cocci are round bacteria, while bacilli are rod-shaped. Bacteria may arrange as single cells, pairs, chains, or clusters, and these morphological features aid in bacterial classification and identification.

Question 7

Why is negative staining necessary for capsule visualization?

Accepted Answer

Capsules have a non-ionic composition and tend to repel stains, making simple staining methods ineffective. Negative staining works by coloring the background and cells while leaving the capsule unstained, creating contrast. This approach avoids heat-fixing, which could damage the capsule and produce false results when using pure cultures and streak plating isolation of single bacterial colonies.

Microscopie et coloration : Gram, Capsule et endospores. - Quiz d'évaluation