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.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
A. A thin peptidoglycan layer: A thin peptidoglycan layer is characteristic of Gram-negative bacteria, not Gram-positive bacteria. In Gram-negative cells, the thin layer is located between the inner cytoplasmic membrane and the outer membrane.
B. A thick peptidoglycan layer: Gram-positive bacteria have a thick, multilayered peptidoglycan wall that provides structural strength, maintains cell shape, and protects against osmotic pressure. This thick layer also traps the crystal violet-iodine complex during Gram staining, giving Gram-positive cells their purple color under a microscope.
C. No peptidoglycan: All bacteria except mycoplasmas have peptidoglycan in their cell walls. Gram-positive bacteria rely heavily on this thick peptidoglycan layer for structural integrity, so that they have no peptidoglycan is incorrect.
D. An outer membrane only: Gram-positive bacteria lack an outer membrane. The presence of an outer membrane is a defining feature of Gram-negative bacteria, which lies outside their thin peptidoglycan layer. Gram-positive bacteria instead have a thick peptidoglycan wall often containing teichoic acids.
Correct Answer is B
Explanation
A. It enhances growth of Gram-positive bacteria: EMB agar does not promote the growth of Gram-positive bacteria; in fact, it inhibits them. Its formulation is designed to favor Gram-negative organisms, particularly enteric bacteria, by incorporating selective agents.
B. It contains dyes that inhibit Gram-positive organisms: EMB (Eosin Methylene Blue) agar contains the dyes eosin Y and methylene blue, which selectively inhibit the growth of most Gram-positive bacteria while allowing Gram-negative bacteria to grow. This selectivity makes EMB useful for isolating enteric Gram-negative bacilli and differentiating lactose fermenters from non-fermenters.
C. It only supports anaerobic growth: EMB agar supports the growth of facultative and aerobic Gram-negative bacteria. Anaerobic growth is not a specific feature of EMB, and oxygen is not restricted in its use.
D. It contains high salt concentration: High salt concentration is a characteristic of selective media like Mannitol Salt Agar (MSA), which inhibits non-halotolerant bacteria. EMB’s selectivity is based on dyes, not salt concentration, so this is not the mechanism for inhibiting Gram-positive bacteria.
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