What characterizes Holt-Harris EMB medium?

Holt-Harris EMB medium is specifically designed for the isolation and differentiation of gram-negative bacteria, particularly those in the family Enterobacteriaceae. The medium contains the indicators Eosin Y and methylene blue, which are crucial for this purpose. These dyes not only help in the selective inhibition of gram-positive bacteria but also aid in the identification of lactose fermenters. When lactose fermenters grow on this medium, they tend to produce acid, which interacts with the indicators to create a distinct color change.

This characteristic makes Eosin Y and methylene blue essential components of the Holt-Harris EMB medium. The presence of these dyes allows for the differentiation of lactose fermenting bacteria, which typically produce dark purple colonies, while non-lactose fermenters remain colorless or take on the color of the medium.

Other options suggest alternative indicators or components that are not found in Holt-Harris EMB medium, such as bromthymol blue or restricting the sugar to only lactose, which does not accurately represent the medium's composition. Additionally, the reference to only producing colorless colonies fails to recognize the range of colony colors indicative of different bacterial behavior on this medium.