TRYPTOPHAN MEDIUM

Tryptophan Medium is prepared according to the formula approved by the ISO Committee, which is a modification of the original APHA formulation. It is used for the detection of indole production, a biochemical marker that is used to differentiate coliforms, especially Enterohemorrhagic Escherichia coli ( EHEC ), including the prototype serotype E. coli O157:H7.
EHEC infections, commonly associated with undercooked ground beef and waterborne or foodborne transmission, can range from asymptomatic carriage to hemorrhagic colitis marked by abdominal cramps and bloody diarrhea. Accurate laboratory identification of E. coli O157:H7 involves a multi-step process in which samples are enriched in Modified Soyabean Bile Broth Base and isolated on MacConkey Sorbitol Agar Base, with presumptive E. coli O157:H7 identified by pale, sorbitol-non-fermenting colonies after incubation. The presumptive colonies are then confirmed by the indole test in Tryptophane Medium.
The medium contains tryptophan, an amino acid substrate which some organisms can convert to indole using the enzyme tryptophanase. Indole reacts with the aldehyde group in the Kovac’s or Ehrlich’s reagent to form a red-colored complex that is concentrated in the alcohol layer, thus giving a positive result. The casein enzymic hydrolysate supplies the necessary carbon and nitrogen sources for the growth of the organism before testing.

Tryptophan Medium is used for detection of indole production