Chromogenic Coliform Agar (CCA)
This is one of the most stunning microbiological tests. CCA (Chromogenic Coliform Agar) is a selective and differential medium used primarily for the simultaneous detection and enumeration of Escherichia coli and coliform bacteria in water, food, and environmental samples. Here’s the principle behind CCA:
Principle of Chromogenic Coliform Agar (CCA)
Selective Components
– CCA contains bile salts and other selective agents that inhibit the growth of Gram-positive bacteria, allowing the selective isolation of Gram-negative bacteria, particularly coliforms.
Differential Components
– Chromogenic Substrates: The key feature of CCA is its inclusion of specific chromogenic substrates that are metabolized by enzymes produced by coliforms and E. coli. These substrates release colored compounds upon hydrolysis, which allows for visual differentiation between species.
– Salmon Gal: This substrate is cleaved by the enzyme β-galactosidase, which is produced by coliform bacteria. When metabolized, it forms salmon-pink colored colonies, indicating the presence of general coliforms.
– X-Glucuronide (X-Gluc): This substrate is cleaved by the enzyme β-glucuronidase, which is produced by E. coli. When metabolized, it forms blue-green colored colonies, indicating the presence of E. coli.
Interpretation
– E. coli Colonies: Colonies that appear blue-green due to the hydrolysis of X-Gluc indicate the presence of E. coli.
– Other Coliform Colonies: Colonies that appear salmon-pink due to the hydrolysis of Salmon-Gal indicate the presence of other coliform bacteria.
– Non-Coliforms: If any non-coliform bacteria grow, they usually appear colorless or take on a different, non-specific color due to the lack of enzyme activity on the chromogenic substrates.
Applications
CCA is widely used in water quality testing and other microbiological analyses where the presence of coliforms and E. coli is a concern. Its ability to differentiate between E. coli and other coliforms in a single medium makes it a valuable tool for rapid and accurate microbial analysis.
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