Why must the smear be completely air-dried before heat fixing?
To prevent distortion of cells
To speed up staining
To make the bacteria grow
To increase contrast
The Correct Answer is A
A. To prevent distortion of cells: Allowing the smear to air-dry completely ensures that the bacterial cells retain their natural shape and size. If the smear is heat-fixed while still wet, the water can rapidly boil, causing cells to rupture, shrink, or burst, which distorts their morphology. Maintaining cell integrity is essential for accurate microscopic examination and identification of bacteria.
B. To speed up staining: Air-drying is not intended to speed up staining; in fact, it is a preparatory step to preserve cell structure before applying stains. Proper drying ensures that stains adhere evenly without artifacts caused by residual moisture.
C. To make the bacteria grow: Smear preparation does not promote bacterial growth. Heat fixing kills the bacteria, making them safe to handle and fixing them to the slide. Growth occurs only on appropriate culture media, not on a prepared smear.
D. To increase contrast: Contrast is primarily achieved through the staining process, such as Gram staining or simple stains. Air-drying itself does not enhance contrast; it only prepares the cells to be fixed and stained properly without distortion.
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Related Questions
Correct Answer is B
Explanation
A. Enzymes that inhibit bacterial growth: Hemolysins do not inhibit bacterial growth; instead, they act on host cells. They are not antimicrobial agents but rather virulence factors that facilitate bacterial survival and spread within the host.
B. Toxins that lyse red blood cells: Hemolysins are exotoxins produced by certain bacteria, such as Streptococcus and Staphylococcus species. They disrupt the membrane of red blood cells, causing hemolysis and releasing hemoglobin. This action provides bacteria with access to nutrients, particularly iron, which is essential for their metabolism and pathogenicity.
C. Structural proteins in bacterial capsules: Capsules are composed of polysaccharides or proteins that protect bacteria from phagocytosis. Hemolysins are not structural components of capsules; they are secreted proteins that damage host cells rather than forming protective layers around the bacterium.
D. Antibiotic resistance factors: Hemolysins do not confer resistance to antibiotics. Antibiotic resistance factors include enzymes such as beta-lactamases or efflux pumps that prevent drugs from killing the bacteria. Hemolysins function solely as cytotoxins targeting host cells.
Correct Answer is B
Explanation
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.
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