Which medium would differentiate Gram-negative from Gram-positive organisms in wound cultures?

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Multiple Choice

Which medium would differentiate Gram-negative from Gram-positive organisms in wound cultures?

Using a selective medium to separate Gram-positive from Gram-negative bacteria in wound specimens is the idea behind this question. Columbia CNA agar contains antibiotics colistin and nalidixic acid that suppress most Gram-negative organisms while allowing Gram-positive bacteria to grow under aerobic conditions for about 24 hours. Because only Gram-positive organisms form colonies, you can distinguish the two groups directly on the plate, which is especially useful in mixed wound cultures where you want a quick, practical separation. Non-selective media like blood agar or chocolate agar won’t separate by Gram status. MacConkey agar, while it helps identify Gram-negative grows and differentiates lactose fermenters, primarily selects for Gram-negatives and inhibits Gram-positives, so it’s a different approach to differentiation.

Using a selective medium to separate Gram-positive from Gram-negative bacteria in wound specimens is the idea behind this question. Columbia CNA agar contains antibiotics colistin and nalidixic acid that suppress most Gram-negative organisms while allowing Gram-positive bacteria to grow under aerobic conditions for about 24 hours. Because only Gram-positive organisms form colonies, you can distinguish the two groups directly on the plate, which is especially useful in mixed wound cultures where you want a quick, practical separation.

Non-selective media like blood agar or chocolate agar won’t separate by Gram status. MacConkey agar, while it helps identify Gram-negative grows and differentiates lactose fermenters, primarily selects for Gram-negatives and inhibits Gram-positives, so it’s a different approach to differentiation.

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