Purpose: TSI agar can be considered an initial step in the identification of the Enterobacteriaceae Salmonella typhi Escherichia coli Citrobacter diversus( Enterobacteriaceae, Salmonella typhi, Escherichia coli, Citrobacter diversus )

Principle: for Indole Broth Urea Agar Voges-proskauer Methyl red( Indole Broth, Urea Agar, Voges-proskauer, Methyl red ): tryptophan PRESENT in peptone is oxidized y certain bacteria to indole, skatole, and indoleacetic acid. Indole is detected in broth cultures of bacteria with an alcoholic p-dimethylaminobenzldehyde reagent. Indole reacts with the aldehyde to form a red black yellow gray( red, black, yellow, gray ) product.

Key Ingredients: Indole broth Methyl Red Phenylalanine LIA( Indole broth, Methyl Red, Phenylalanine, LIA ): Tryptophan (1% broth), Kovac's reagent or Ehrlich's reagent, Xylene or chloroform for extraction if using Ehrlich's reagent

Indole Broth Reactions:
Negative Reaction: No Red Dark Yellow( No, Red, Dark, Yellow ) color development
Positive Reaction: red yellow no brown( red, yellow, no, brown ) ring at the interfacec of reagent and broth)
Variable reaction: Orange Red Black Purple( Orange, Red, Black, Purple ) color, indicates production of aktole, a methylated intermediate that may be a precursor to indole production

Principle: Urea Hydrolysis (Christensen's Agar): Many organisms possess the enzyme urease which can hydrolyze urea with the release of ammonia and carbon dioxide which acts to increase the pH of the medium and turn the indicator phenol red from pale orange fuchsia pink violet purple green( orange, fuchsia pink, violet purple, green ) to dark fuchsia pink orange violet purple green( fuchsia pink, orange, violet purple, green )

Key Ingredients: Urea Agar
Urea Agar (christensen's Base methyl red spot indole broth citrate( christensen's Base, methyl red, spot indole broth, citrate ))
pH indicator = phenol red methyl red gelatin API 20E strip( phenol red, methyl red, gelatin, API 20E strip )

Interpretation: urea hydrolysis
A strong positive hydrolysis test: entire agar, slant and butt, turning deep pink deep purple light orange light pink( deep pink, deep purple, light orange, light pink ) after overnight incubation.
A moderate positive test: pink orange yellow red( pink, orange, yellow, red ) slant at 24 hours, with the intensity of the color increasing with extended incubation
A negative test: no change from pale orange pink( orange, pink ) color of the slant