SalmonellaShigella

SalmonellaShigella is a differential growth medium used in microbiology laboratories. It is designed to isolate gram-negative bacteria from gram-positive bacteria, and to differentiate between lactose fermenters and non-lactose fermenters. The medium contains several key ingredients that contribute to its selective and differential properties. It includes bile salts and brilliant green dye, which inhibit the growth of most gram-positive bacteria and some gram-negative bacteria. Sodium thiosulfate and ferrous sulfate are also present, allowing for the detection of hydrogen sulfide production, which is indicated by a black precipitate. Lactose is included as a fermentable carbohydrate. Bacteria that ferment lactose produce acid, which lowers the pH of the medium. This change in pH is often visualized by a color indicator, such as phenol red, which turns yellow in acidic conditions. Therefore, lactose fermenters will typically appear as red or pink colonies, while non-lactose fermenters will appear as colorless or greenish colonies. Salmonella species are generally non-lactose fermenters and produce hydrogen sulfide, resulting in black colonies. Shigella species are also non-lactose fermenters but do not typically produce hydrogen sulfide. The medium is commonly used for the isolation of enteric pathogens from clinical specimens and food samples.