Organic Chemistry

Probability

Eosin Methylene Blue (EMB) agar is a selective and differential culture medium used primarily for isolating fecal coliforms. It is selective because it inhibits the growth of Gram-positive bacteria, and it is differential because it distinguishes between lactose fermenters and non-lactose fermenters among the Gram-negative bacteria. The medium contains lactose as a fermentable carbohydrate, and the dyes eosin Y and methylene blue, which play crucial roles in the differentiation process. Here is a step-by-step explanation of how EMB agar differentiates between Gram-negative bacteria based on their ability to ferment lactose: 1. **Selective Action**: The dyes eosin Y and methylene blue are toxic to Gram-positive bacteria due to their ability to inhibit the growth of these organisms. This selectivity is based on the differences in the cell wall structure between Gram-positive and Gram-negative bacteria. Gram-positive bacteria have a thick peptidoglycan layer that is disrupted by these dyes, whereas Gram-negative bacteria have an outer membrane that provides some protection against the dyes. 2. **Differential Action**: The differential aspect of EMB agar is based on the ability of bacteria to ferment lactose present in the medium. When a Gram-negative bacterium ferments lactose, it produces acidic byproducts. The acidity from the fermentation lowers the pH of the medium surrounding the colonies. 3. **Role of Dyes in pH Indicator**: Eosin Y and methylene blue act as pH indicators. At neutral pH, these dyes form a complex that is colorless or has a very light color. However, when the pH drops due to the production of acid from lactose fermentation, the dyes undergo a color change. 4. **Color Change and Metallic Green Sheen**: Vigorous lactose fermenters, such as Escherichia coli, produce a significant amount of acid, which leads to a very low pH. This results in the absorption of the dyes into the bacterial colonies, turning them a dark purple or black color. In some cases, the colonies may even exhibit a metallic green sheen due to the strong acid production and dye absorption. 5. **Non-Lactose Fermenters**: Bacteria that do not ferment lactose do not produce acid and therefore do not cause a significant pH change in the medium. As a result, these colonies typically remain colorless or take on the color of the medium, which is usually a light purple or pink due to the slight acidity of the agar itself. In summary, EMB agar uses the dyes eosin Y and methylene blue to inhibit the growth of Gram-positive bacteria and to differentiate between lactose fermenters and non-lactose fermenters among Gram-negative bacteria. Lactose fermenters produce acid, leading to a pH change that results in the absorption of the dyes and a color change in the colonies, while non-lactose fermenters do not produce acid and thus do not cause a significant color change.

Biology

How does eosin methylene blue agar (EMB) differentiate between Gram-negative bacteria, and what role do the dyes eosin and methylene blue play in identifying lactose fermentation?

Question

How does eosin methylene blue agar (EMB) differentiate between Gram-negative bacteria, and what role...

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