What modification is made to MacConkey agar when used for Mycobacterium identification?

What modification is made to MacConkey agar when used for Mycobacterium identification?

• A. Add crystal violet
• B. Add bile salts
• C. Omit crystal violet
• D. Add lactose

Answer: (C)

MacConkey agar is designed to suppress Gram-positive bacteria using bile salts and crystal violet, while allowing many Gram-negative bacteria to grow and differentiating lactose fermenters. Some Mycobacterium species are inhibited by crystal violet, so removing that dye from the medium lets these organisms grow more readily on a MacConkey-type medium. The other components, like lactose, are not the modification needed for identifying Mycobacterium, so the key change is omitting crystal violet.

MacConkey agar is designed to suppress Gram-positive bacteria using bile salts and crystal violet, while allowing many Gram-negative bacteria to grow and differentiating lactose fermenters. Some Mycobacterium species are inhibited by crystal violet, so removing that dye from the medium lets these organisms grow more readily on a MacConkey-type medium. The other components, like lactose, are not the modification needed for identifying Mycobacterium, so the key change is omitting crystal violet.