Apocholate citrate agar

Apocholate Citrate Agar

Apocholate citrate agar is a specialized culture medium used in microbiology for the selective isolation and differentiation of certain Enterobacteriaceae, particularly those that are lactose-fermenting and non-lactose-fermenting. This medium is particularly useful in the identification of Salmonella and Shigella species from clinical and environmental samples.

Composition

Apocholate citrate agar contains several key ingredients that contribute to its selective and differential properties:

• Bile salts: These inhibit the growth of most Gram-positive organisms, allowing for the selective growth of Gram-negative bacteria.
• Sodium citrate: Acts as a selective agent by inhibiting certain Gram-negative bacteria that are sensitive to citrate.
• Lactose: Serves as a fermentable carbohydrate source, allowing for differentiation based on lactose fermentation.
• Neutral red: A pH indicator that changes color in response to acid production from lactose fermentation.
• Agar: Provides a solid matrix for bacterial growth.

Mechanism of Action

The selective nature of apocholate citrate agar is primarily due to the presence of bile salts and sodium citrate, which inhibit the growth of unwanted bacteria. The differential aspect is achieved through the inclusion of lactose and neutral red. Bacteria that ferment lactose produce acid, leading to a color change in the medium due to the pH indicator. Lactose fermenters typically appear as red or pink colonies, while non-lactose fermenters remain colorless or take on the natural color of the medium.

Applications

Apocholate citrate agar is widely used in clinical laboratories for the isolation of pathogenic enteric bacteria. It is particularly useful in stool cultures where the presence of Salmonella and Shigella needs to be determined. The medium's ability to differentiate between lactose fermenters and non-fermenters aids in the preliminary identification of these pathogens.

Limitations

While apocholate citrate agar is effective for the isolation of certain pathogens, it is not suitable for all types of bacteria. Some strains of Escherichia coli and other lactose fermenters may also grow on this medium, potentially complicating the identification process. Additional confirmatory tests are often required to accurately identify the isolated organisms.

Also see

• MacConkey agar
• Hektoen enteric agar
• Xylose lysine deoxycholate agar
• Microbiological culture