Bacteria growing on eosin methylene blue (EMB) agar will most likely be Gram:

• A. Variable
• B. Negative
• C. Positive
• D. Neither positive nor negative

Answer: (B)

Verified step by step guidance

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.