Question 1

What is the difference between Gram-positive and Gram-negative bacterial cell walls?

Accepted Answer

Gram-positive bacteria have a thick peptidoglycan layer that contracts during decolorization, trapping crystal violet-iodine complexes inside the cell wall. Gram-negative bacteria possess a thinner peptidoglycan layer sandwiched between two membranes, allowing the decolorizing agent to compromise the outer membrane and wash away the dye complexes, resulting in different colors under microscopy.

Question 2

How does crystal violet bind to bacterial cells during Gram staining?

Accepted Answer

Positively-charged crystal violet binds weakly to negatively-charged bacterial cell walls. Gram's iodine then forms an insoluble complex with the crystal violet dye, fixing it within the cell wall structure and enabling visualization under microscopy. This complex formation is essential for the staining process to work effectively.

Question 3

What steps are involved in preparing a bacterial smear for Gram staining?

Accepted Answer

Mix a small amount of isolated bacterial culture with sterile distilled water on a glass slide to create a diluted milk-like consistency. Allow the smear to dry at room temperature, then heat-fix it by quickly passing the slide through a flame to adhere bacteria to the glass surface permanently.

Question 4

Why is safranin applied as a counterstain in the Gram staining procedure?

Accepted Answer

Safranin is applied after decolorization to visualize Gram-negative bacteria, which appear red or pink under microscopy. This counterstain ensures that both Gram-positive cells appearing blue or purple and Gram-negative cells are visible and distinguishable in the final preparation for accurate bacterial classification.

Question 5

How are bacterial colonies isolated from environmental soil samples?

Accepted Answer

Soil samples are diluted serially and plated onto low-nutrient agarose media. After incubation, colonies from the highest dilution with 30 to 300 discrete colonies are selected and streaked onto fresh plates in a zig-zag pattern to obtain isolated single colonies for culturing and enumerating bacteria from soil samples.

Question 6

What information can be determined from observing Gram-stained bacterial cells under a microscope?

Accepted Answer

Gram staining reveals bacterial morphology, including cell shape, size, and arrangement such as chains or clusters. Color indicates Gram classification: blue or purple for Gram-positive cells and red or pink for Gram-negative cells, enabling rapid categorization of bacteria into two broad structural classes.

Question 7

How does Gram staining help identify bacteria in clinical and environmental applications?

Accepted Answer

Gram staining provides rapid visual distinction of bacterial populations, enabling targeted identification strategies. For example, knowing a bacterium is Gram-positive allows selection of species-specific peptide nucleic acid probes or selective media like cycloserine agar that inhibits Gram-negative growth, facilitating isolation of bacteria of interest.

Gram Staining of Bacteria from Environmental Sources - Assessment Quiz