Why is heat fixing performed during smear preparation?
To stain the bacteria
To kill bacteria and adhere them to the slide
To increase magnification
To remove excess stain
The Correct Answer is B
A. To stain the bacteria: Heat fixing does not apply stain to the bacteria; staining is a separate step performed after the smear is fixed. Heat fixing prepares the cells to better accept the stain by immobilizing them and preserving their structure.
B. To kill bacteria and adhere them to the slide: Heat fixing serves two main purposes: it kills the bacteria, making the slide safe to handle, and it causes the proteins in the cells to coagulate slightly, which adheres the cells firmly to the glass slide. This prevents them from washing off during the staining and rinsing and preserves their morphology for accurate examination.
C. To increase magnification: Heat fixing has no effect on the magnification of the microscope. Magnification is controlled by the objective lenses and ocular lenses, not by the preparation technique of the smear.
D. To remove excess stain: Heat fixing does not remove stain; rather, it prepares the cells so that stains can bind effectively. Removal of excess stain is achieved through rinsing with water or appropriate solvents after staining, not through heat fixation.
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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. 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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