The choices describe possible scenarios and outcomes from Gram staining with standard stains (e.g., crystal violet and safranin). Match the scenario with the most likely outcome and explain why this occurs.
What are your choices?
A. A gram-negative cell is stained, but the ethanol step is skipped.
B. A gram-positive cell with a severely damaged cell wall is stained.
C. A healthy gram-negative cell is stained.
D. A healthy gram-positive cell is stained.
i. The cell will probably appear pink, which will not correctly indicate its cell wall structure.
ii. The cell will appear pink.
iii. The cell will appear purple/violet.
iv. The cell will probably appear purple/violet, which will not correctly indicate its cell wall structure.
To match the scenarios with the most likely outcomes from Gram staining with standard stains, you'll need to understand Gram staining and the differences between Gram-positive and Gram-negative bacteria.
Gram staining is a common technique used to differentiate between different types of bacteria. It involves staining the bacteria with crystal violet, followed by iodine, alcohol, and a counterstain such as safranin. The staining process helps to classify bacteria into two major groups: Gram-positive and Gram-negative.
Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, while Gram-negative bacteria have a thinner peptidoglycan layer and an additional outer membrane. These structural differences result in differences in staining properties.
Now, let's discuss the possible scenarios and their most likely outcomes:
1. Scenario: Gram-positive bacteria stained with crystal violet.
Most likely outcome: Gram-positive bacteria would retain the crystal violet stain even after the decolorization with alcohol.
Explanation: The thick peptidoglycan layer in Gram-positive bacteria traps the crystal violet-iodine complex, allowing the bacteria to retain the purple color after the alcohol decolorization step. The subsequent counterstain with safranin does not affect the purple color, and Gram-positive bacteria appear as purple under a microscope.
2. Scenario: Gram-negative bacteria stained with crystal violet.
Most likely outcome: Gram-negative bacteria would lose the crystal violet stain after the decolorization step and take up the safranin counterstain.
Explanation: The thinner peptidoglycan layer of Gram-negative bacteria cannot retain the crystal violet-iodine complex. Therefore, during the alcohol decolorization step, the crystal violet is washed out. The safranin counterstain that follows stains the Gram-negative bacteria pink or red, making them appear pink under a microscope.
3. Scenario: Gram-positive bacteria stained with safranin.
Most likely outcome: Gram-positive bacteria would stain pink after the safranin counterstain.
Explanation: Since the initial stain used is safranin instead of crystal violet, both Gram-positive and Gram-negative bacteria would appear pink after staining. However, due to the thicker peptidoglycan layer of Gram-positive bacteria, they retain the pink safranin stain more strongly and appear darker pink under a microscope.
4. Scenario: Gram-negative bacteria stained with safranin.
Most likely outcome: Gram-negative bacteria would stain pink after the safranin counterstain.
Explanation: Similar to the previous scenario, both Gram-positive and Gram-negative bacteria would stain pink after staining with safranin. However, due to the thinner peptidoglycan layer and the additional outer membrane of Gram-negative bacteria, they retain the pink safranin stain less strongly and appear lighter pink or pale under a microscope.
Remember, these outcomes are based on the general characteristics of Gram-positive and Gram-negative bacteria and their response to crystal violet and safranin stains. It's important to note that there can be variations and exceptions in bacterial staining, so microscopic examination is crucial for accurate identification.