Gram-negative bacteria differ from Gram-positive bacteria because they have:
A thick peptidoglycan layer
No cell wall
An outer membrane containing lipopolysaccharide (LPS)
Teichoic acids in their cell wall
The Correct Answer is C
A. A thick peptidoglycan layer: Gram-positive bacteria have a thick peptidoglycan layer, which retains the crystal violet-iodine complex during Gram staining. Gram-negative bacteria have a much thinner peptidoglycan layer, which does not retain the stain after decolorization.
B. No cell wall: Gram-negative bacteria do have a cell wall; it is just structurally different from Gram-positive bacteria. Their wall consists of a thin peptidoglycan layer sandwiched between the inner cytoplasmic membrane and an outer membrane containing lipopolysaccharides.
C. An outer membrane containing lipopolysaccharide (LPS): Gram-negative bacteria possess an outer membrane external to the peptidoglycan layer. This outer membrane contains LPS, which contributes to the structural integrity of the cell, acts as an endotoxin, and protects against certain antibiotics and host defenses. The presence of LPS is a key distinguishing feature.
D. Teichoic acids in their cell wall: Teichoic acids are found only in the cell walls of Gram-positive bacteria. They provide structural support, contribute to cell wall maintenance, and help with adherence, but are absent in Gram-negative bacteria.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is C
Explanation
A. Purple: Purple indicates Gram-positive bacteria, which retain the crystal violet-iodine complex within their thick peptidoglycan cell walls during the Gram staining process. The color persists even after decolorization with alcohol or acetone.
B. Blue: Blue is not a standard color outcome in Gram staining. Gram-negative and Gram-positive bacteria are typically differentiated as purple or pink/red; blue may appear in other specialized staining techniques but not standard Gram staining.
C. Pink/Red: Gram-negative bacteria appear pink or red after Gram staining. Their thin peptidoglycan layer does not retain the crystal violet-iodine complex after decolorization. They are counterstained with safranin, which imparts the pink/red color, allowing differentiation from Gram-positive bacteria.
D. Green: Green is not a result of Gram staining. Green coloration is associated with other staining methods or pigments but is not part of the Gram stain differential process.
Correct Answer is A
Explanation
Alpha hemolysis is characterized by partial lysis of red blood cells on blood agar, leading to a greenish or brownish discoloration around bacterial colonies rather than a clear zone. This occurs because the bacteria produce hemolysins that oxidize hemoglobin to methemoglobin, causing the greenish hue, but do not completely lyse the red blood cells. In contrast, beta hemolysis results in complete destruction of red blood cells and a clear zone surrounding the colonies. Therefore, alpha hemolysis produces incomplete hemolysis with color change rather than full destruction and clearing of the agar.
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