Why does Mannitol Salt Agar inhibit most bacteria except Staphylococcus species?
It contains lactose
It contains 7.5% sodium chloride
It lacks nutrients
It contains bile salts
The Correct Answer is B
A. It contains lactose: Mannitol Salt Agar (MSA) does contain mannitol as a fermentable carbohydrate to differentiate Staphylococcus species, but lactose is not part of the medium. Lactose would not selectively inhibit most bacteria; it is primarily used for fermentation detection in other types of media like MacConkey agar.
B. It contains 7.5% sodium chloride: MSA contains a high concentration of sodium chloride (7.5%), which creates a hypertonic environment that inhibits the growth of most bacteria that are not salt-tolerant. Staphylococcus species are halotolerant, meaning they can survive and grow in high-salt conditions. This selective property allows Staphylococcus to be distinguished from other bacteria in mixed cultures.
C. It lacks nutrients: MSA is a nutrient-rich medium containing peptones and other growth factors. Its selectivity is not due to nutrient deprivation but rather the high salt content that inhibits non-halotolerant bacteria.
D. It contains bile salts: Bile salts are used in selective media like MacConkey agar to inhibit Gram-positive bacteria. MSA does not contain bile salts; its selectivity is based on sodium chloride, not bile.
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Naxlex Comprehensive Predictor Exams
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 A
Explanation
Gamma hemolysis, also called non-hemolysis, refers to bacterial growth on blood agar that does not cause any breakdown of red blood cells. On a blood agar plate, colonies exhibiting gamma hemolysis appear unchanged, with no clear or greenish zones surrounding them, indicating the bacteria do not produce hemolysins that lyse erythrocytes. This contrasts with alpha hemolysis, which causes partial hemolysis and greenish discoloration, and beta hemolysis, which produces complete lysis and clear zones around colonies. Therefore, gamma hemolysis signifies the absence of red blood cell lysis.
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