Which component of EMB agar is primarily responsible for inhibiting Gram-positive bacteria?
Lactose
Agar
Eosin and methylene blue dyes
Sodium chloride
The Correct Answer is C
A. Lactose: Lactose in Eosin Methylene Blue (EMB) agar serves as a fermentable carbohydrate source. It allows differentiation of bacteria based on their ability to ferment lactose, producing color changes in the colonies, but it does not inhibit Gram-positive bacteria.
B. Agar: Agar is a solidifying agent that provides a stable surface for microbial growth. It is inert and does not selectively inhibit any bacterial group; its role is purely structural.
C. Eosin and methylene blue dyes: The eosin and methylene blue dyes in EMB agar act as selective agents that inhibit the growth of Gram-positive bacteria while allowing Gram-negative bacteria to grow. These dyes also serve as pH indicators, producing color changes in colonies that ferment lactose, which aids in differentiation of coliforms and other Gram-negative bacteria.
D. Sodium chloride: Sodium chloride maintains osmotic balance in the medium but does not selectively inhibit Gram-positive bacteria. Its concentration is not sufficient to act as a selective agent in EMB agar.
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Related Questions
Correct Answer is C
Explanation
A. Pink: Pink indicates Gram-negative bacteria, which do not retain the crystal violet stain during the decolorization step and are counterstained with safranin. Gram-positive bacteria are not pink after Gram staining.
B. Red: Red is another way to describe the color of Gram-negative bacteria after counterstaining with safranin. Gram-positive bacteria do not take up the red counterstain because they retain the primary crystal violet stain.
C. Purple: Gram-positive bacteria appear purple after Gram staining. Their thick peptidoglycan cell walls trap the crystal violet-iodine complex even after alcohol or acetone decolorization, allowing them to retain the purple color and be distinguished from Gram-negative bacteria.
D. Colorless: Colorless cells would indicate a failure to stain or a decayed sample. Gram-positive bacteria are never colorless after a proper Gram staining procedure.
Correct Answer is C
Explanation
A. 40x: 40x represents only the magnification of the objective lens alone. Total magnification is calculated by multiplying the objective lens magnification by the eyepiece magnification, so 40x alone does not reflect the combined power.
B. 100x: 100x would result from a 10x eyepiece combined with a 10x objective lens, not a 40x objective. Using the wrong combination for calculation would underestimate the actual total magnification.
C. 400x: Total magnification is calculated by multiplying the eyepiece (10x) by the objective lens (40x): 10 × 40 = 400. This total magnification allows clear visualization of medium-sized bacterial cells under the light microscope.
D. 4,000x: 4,000x would be achieved with much higher objective lenses, such as an oil immersion 400x objective combined with a 10x eyepiece. Using 40x for the objective does not reach this level of magnification.
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