The colourless colonies are only the gram-negative bacterium that will grow as the selective media Eosin Methylene Blue (EMB) followed by mannitol salt agar (MSA), which will only allow one type of microorganism to grow.
Eosin Methylene Blue (EMB) agar is a differential microbiological medium that gives a colour indicator to distinguish between organisms that digest lactose and those that do not while significantly inhibiting the development of Gram-positive bacteria. In laboratories, the selective medium is crucial for differentiating gram-negative microorganisms.
Both EMB (Eosin methylene blue) and Mannitol Salt Agar (MSA) are selective in that they only permit the growth of Gram-negative bacteria.
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