Multiple Choice
If you forget to use the decolorizer during a Gram stain, which cell type(s) will appear purple under the microscope?
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7. Prokaryotic Cell Structures & Functions
Gram-Positive vs. Gram-Negative Cell Walls
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Bacteria growing on eosin methylene blue (EMB) agar will most likely be Gram:
A
Variable
B
Negative
C
Positive
D
Neither positive nor negative
Verified step by step guidance1
Understand the purpose of eosin methylene blue (EMB) agar: EMB agar is a selective and differential medium primarily used to isolate Gram-negative bacteria, especially members of the Enterobacteriaceae family.
Recall the selectivity of EMB agar: The dyes eosin Y and methylene blue inhibit the growth of Gram-positive bacteria, allowing Gram-negative bacteria to grow more readily on this medium.
Recognize the differential aspect: EMB agar differentiates bacteria based on their ability to ferment lactose, but this does not affect the Gram reaction; it only helps identify lactose fermenters among Gram-negative bacteria.
Conclude the Gram reaction of bacteria growing on EMB agar: Since EMB agar inhibits Gram-positive bacteria, the bacteria that grow on it are most likely Gram-negative.
Summarize: Therefore, bacteria growing on EMB agar are typically Gram-negative due to the selective inhibition of Gram-positive organisms by the dyes in the medium.
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