DCA agar

DCA Agar

DCA agar, or Deoxycholate Citrate Agar, is a selective and differential culture medium primarily used for the isolation of enteric pathogens, particularly members of the genus Salmonella and Shigella. This medium is designed to inhibit the growth of Gram-positive bacteria and some Gram-negative bacteria, allowing for the selective growth of enteric pathogens.

Composition

DCA agar contains several key components that contribute to its selective and differential properties:

• Peptone: Provides essential nutrients for bacterial growth.
• Lactose: A carbohydrate source that allows for differentiation based on lactose fermentation.
• Sodium deoxycholate: A bile salt that inhibits the growth of Gram-positive bacteria and some non-enteric Gram-negative bacteria.
• Sodium citrate: Enhances the selectivity of the medium by inhibiting certain bacteria.
• Neutral red: A pH indicator that changes color in response to acid production from lactose fermentation.
• Ferric ammonium citrate: Reacts with hydrogen sulfide to produce black precipitates, indicating the presence of hydrogen sulfide-producing bacteria.
• Agar: A solidifying agent.

Mechanism of Action

DCA agar works by exploiting the metabolic differences between enteric pathogens and other bacteria. The bile salts and sodium citrate inhibit the growth of non-target organisms, while the lactose and neutral red allow for differentiation based on lactose fermentation. Salmonella and Shigella do not ferment lactose, so they typically produce colorless colonies. In contrast, lactose fermenters such as Escherichia coli produce pink colonies due to acid production, which lowers the pH and changes the color of the neutral red indicator.

Applications

DCA agar is widely used in clinical microbiology laboratories for the isolation and identification of Salmonella and Shigella from stool samples, food products, and environmental samples. It is particularly useful in the diagnosis of enteric fevers and dysentery.

Limitations

While DCA agar is effective for isolating Salmonella and Shigella, it is not entirely specific, and some strains of other bacteria may grow on this medium. Additionally, some strains of Salmonella may produce hydrogen sulfide, resulting in black-centered colonies, which can be confused with other hydrogen sulfide-producing bacteria.

Also see

• MacConkey agar
• XLD agar
• Hektoen enteric agar
• Microbiological culture
• Selective media

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