The Gram staining test divides species of bacteria into Gram-positive as well as Gram-negative groups. In the test, gram-positive bacteria produce a purple color, whereas, gram-negative bacteria produce a pink tint.
Gram-negative bacteria would normally stain pink throughout the process, although Enterobacter aerogenes already possess a purplish hue due to being cultivated in an EMB plate, hence they shouldn't be Gram stained.
Eosin Methylene Blue agar is present in the EMB plate. Gram-negative bacteria are the only ones that can survive on the plate, hence this test is used to separate them from a sample of other species. Methylene blue, as well as eosin Y, as well as eosin Y is pH indicator dyes. At low pH, they combine to produce a dark purple precipitate.
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