Which of the following is used as selective media for Salmonella and Shigella?

Hektoen Enteric (HE) agar is a selective and differential medium specifically designed to isolate and differentiate enteric gram-negative pathogens, primarily Salmonella and Shigella. The medium contains bile salts that inhibit the growth of gram-positive organisms and non-pathogenic gram-negative bacteria, allowing for the selective isolation of Salmonella and Shigella from clinical specimens or food samples.

In addition to its selectivity, HE agar differentiates between these pathogens based on their biochemical reactions. Salmonella typically produces hydrogen sulfide (H2S), which reacts with iron salts in the agar to form black colonies, while Shigella does not produce H2S, resulting in green colonies. This ability to differentiate between pathogens based on color changes enhances the utility of HE agar in clinical microbiology.

Other media mentioned, such as MacConkey agar, while selective for gram-negative organisms, do not specifically target Salmonella and Shigella and may not be as effective in isolating these pathogens compared to HE agar. Blood agar is a rich, non-selective medium that supports the growth of a wide range of organisms but does not have the selectivity needed for Salmonella and Shigella. Nutrient agar is also non-selective and provides general growth