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Godsey et al. supported their findings in an evaluation of methods based on microbial enzyme activity profiles3. Most strains of E. coli produce an enzyme β-D-glucuronidase. In the indole test, the enzyme tryptophanase attacks tryptophan molecule on its side chain, leaving the aromatic ring in the form of indole. The indole is then detected by addition of ρ-dimethylamino-benzaldehyde (Kavocs’ Reagent) which produces a red color. β-D-glucuronidase is an enzyme that cleaves the substrate 4-methylumbelliferyl-β-D-glucuronide (MUG) and thereby produces a fluorescent product called methylumbelliferone4. This compound is detectable using a long wave ultraviolet light (360 nm).
Interpretation:
MUG Disk Test (Direct or Tube Test):
Positive test - Blue fluorescence observed under 360 nm UV light
Negative test - No fluorescence observed under 360 nm UV light
Indole Test:
Positive test - Red color development on the disk or in the tube
Negative test - No red color development on the disk or in the tube