What makes eosin methylene blue selective?

In this manner, what ingredient makes eosin methylene blue agar selective?

Eosin methylene blue (EMB, also known as "Levine's formulation") is a selective stain for Gram-negative bacteria. EMB contains dyes that are toxic to Gram-positive bacteria. EMB is the selective and differential medium for coliforms. It is a blend of two stains, eosin and methylene blue in the ratio of 6:1.

Likewise, why does EMB inhibit gram positive? Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by-products of fermentation. The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye.

Hereof, what is the purpose of eosin and methylene blue dyes in eosin methylene blue agar?

Eosin methylene blue agar (EMB) is a selective and differential medium used to isolate fecal coliforms. Eosin Y and methylene blue are pH indicator dyes which combine to form a dark purple precipitate at low pH; they also serve to inhibit the growth of most Gram positive organisms.

What specific ingredient makes MacConkey agar selective and which ingredient makes it differential?

MacConkey Agar. This medium is both selective and differential. The selective ingredients are the bile salts and the dye, crystal violet which inhibit the growth of Gram-positive bacteria.