Why do Gram-positive bacteria retain the crystal violet stain?
Their outer membrane traps the stain
Their thick peptidoglycan layer holds the crystal violet-iodine complex
They absorb the safranin more strongly
They do not undergo decolorization
The Correct Answer is B
A. Their outer membrane traps the stain: Gram-positive bacteria do not have an outer membrane; this structure is characteristic of Gram-negative bacteria. Therefore, the outer membrane cannot be responsible for retaining crystal violet in Gram-positive cells.
B. Their thick peptidoglycan layer holds the crystal violet-iodine complex: Gram-positive bacteria have a thick, multilayered peptidoglycan cell wall that forms a dense network. During Gram staining, the crystal violet combines with iodine to form an insoluble complex that gets trapped within this thick peptidoglycan matrix. Even after alcohol or acetone decolorization, the complex remains, causing the cells to appear purple under a microscope.
C. They absorb the safranin more strongly: Safranin is a counterstain used to color Gram-negative bacteria red after decolorization. Gram-positive bacteria do not absorb safranin strongly because the retained crystal violet-iodine complex masks it. Therefore, safranin absorption is not responsible for the purple color of Gram-positive cells.
D. They do not undergo decolorization: While Gram-positive bacteria resist decolorization, this is not because they fail to undergo the process; it is due to the structural ability of their thick peptidoglycan to trap the crystal violet-iodine complex. The resistance is a result of the cell wall structure, not a lack of exposure to the decolorizing agent.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
A blood agar plate is a nutrient-rich culture medium that contains mammalian blood, usually 5% sheep blood, mixed with agar. It is commonly used in microbiology laboratories to grow and differentiate bacteria based on their ability to lyse red blood cells (hemolysis). The red color of the medium comes from the intact red blood cells, and bacterial colonies may produce clear zones (beta hemolysis), greenish zones (alpha hemolysis), or no change (gamma hemolysis) around them. This property helps in identifying clinically important bacteria such as Streptococcus species.
Correct Answer is B
Explanation
A. It contains lactose: Mannitol Salt Agar (MSA) does contain mannitol as a fermentable carbohydrate to differentiate Staphylococcus species, but lactose is not part of the medium. Lactose would not selectively inhibit most bacteria; it is primarily used for fermentation detection in other types of media like MacConkey agar.
B. It contains 7.5% sodium chloride: MSA contains a high concentration of sodium chloride (7.5%), which creates a hypertonic environment that inhibits the growth of most bacteria that are not salt-tolerant. Staphylococcus species are halotolerant, meaning they can survive and grow in high-salt conditions. This selective property allows Staphylococcus to be distinguished from other bacteria in mixed cultures.
C. It lacks nutrients: MSA is a nutrient-rich medium containing peptones and other growth factors. Its selectivity is not due to nutrient deprivation but rather the high salt content that inhibits non-halotolerant bacteria.
D. It contains bile salts: Bile salts are used in selective media like MacConkey agar to inhibit Gram-positive bacteria. MSA does not contain bile salts; its selectivity is based on sodium chloride, not bile.
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