Which of the following dyes is commonly used in simple staining?
Safranin
Methylene blue
Malachite green
Carbol fuchsin
The Correct Answer is B
A. Safranin: Safranin is a basic dye that is primarily used as a counterstain in differential staining techniques such as the Gram stain and endospore staining. It provides contrast to differentiate between Gram-positive and Gram-negative bacteria or to highlight structures like spores, but it is not typically used alone for simple staining.
B. Methylene blue: Methylene blue is a cationic (basic) dye that binds electrostatically to negatively charged components of bacterial cells, such as nucleic acids and cell walls. Its use in simple staining allows clear visualization of cell morphology, size, and arrangement under a light microscope. This makes it a reliable and widely accepted stain in microbiology for rapid identification of bacterial cellular features.
C. Malachite green: Malachite green is a primary stain used in endospore staining procedures because it can penetrate the tough keratin coat of bacterial spores, usually with the aid of heat. It is not suitable for simple staining as it does not readily stain vegetative cells and requires additional steps, making it more specialized.
D. Carbol fuchsin: Carbol fuchsin is a phenolic dye used in acid-fast staining to identify mycobacteria, which have waxy, lipid-rich cell walls resistant to conventional stains. Its medical significance lies in tuberculosis and leprosy diagnostics, but it is not used for simple staining because its application requires heat and decolorization with acid-alcohol.
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Naxlex Comprehensive Predictor Exams
Related Questions
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.
Correct Answer is B
Explanation
The goal of streaking is to isolate individual bacterial colonies from a mixed or dense culture, not to produce a thick, continuous layer. By streaking a loop across an agar plate in a systematic pattern, bacteria are progressively thinned out, allowing single cells to settle separately. These individual cells then grow into discrete colonies that can be identified, counted, or further studied. Producing a thick, continuous layer would prevent isolation and make it difficult to distinguish individual colonies, defeating the purpose of streaking for isolation and accurate microbiological analysis.
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