Tryptophan Solution

Principle And Interpretation

Enterohemorrhagic Escherichia coli (EHEC) is a defined subset of Shiga-like (vero) toxin-producing E. coli. EHEC infections are waterborne or food borne. EHEC is ingested most commonly with undercooked ground beef (1, 2, 3). There are more than 50 serotypes of EHEC. However, E. coli O157:H7 is the prototype EHEC. E. coli O157:H7 can cause an asymptomatic infection, mild diarrhea or a diarrheal illness that is characterized by non-bloody (progressing to bloody) diarrhea and abdominal cramps (together known as hemorrhagic colitis), few leukocytes in stools and lack of significant fever (1, 2,4). Tryptophan Medium is prepared as per the formula approved by ISO Committee (5), that is a modification of original formula of APHA where the medium is devoid of tryptophan (6). This medium is useful for the detection of indole production by Escherichia coli O157: H7 which is a key feature in differentiation of coliforms. Casein enzymic hydrolysate provides carbonaceous and nitrogenous sources required for the growth of microorganisms. Tryptophan is an amino acid, which serves as a substrate to study indole reaction. Certain microorganisms breakdown tryptophan with the help of the enzyme tryptophanase that mediate the production of indole by hydrolytic activity (7). The indole produced can be detected by Kovacs or Ehrlichs reagent (8). Indole combines with the aldehyde present in the above reagent to give red colour in the alcohol layer. The alcohol layer extracts and concentrates the red colour complex. The test sample is enriched in Modified Soyabean Bile Broth Base (M12861) by incubating at 42°C for 18-24 hours. E. coli O157:H7 is then isolated on MacConkey Sorbitol Agar Base (M2981). Pale coloured colonies obtained on incubation at 35-37°C for 18-24 hours are reported as presumptive E. coli O157:H7. Presumptive colonies are subjected to indole test that makes the use of Tryptophan Medium (M1339).