Questions and answers

What does negative indole test mean?

What does negative indole test mean?

Indole Spot Reagent Result Positive reaction: The development of a blue colour within 3 minutes. Negative reaction: The development of a pink colour within 3 minutes.

Is the indole test selective or differential?

This is a differential medium. It tests the ability of an organism to do several things: reduce sulfur, produce indole and swim through the agar (be motile).

What is the indicator for indole test?

To test for the presence of indole, a by-product of tryptophan metabolism, 5 drops of Kovács reagent should be added to the top of the deep. A positive indole test is indicated by the formation of a red color in the reagent layer on top of the agar deep within seconds of adding the reagent.

How is indole production detected?

Indole production is detected by Kovac’s or Ehrlich’s reagent. Indole, if present, combines with the aldehyde in the reagent to produce a pink to red-violet quinoidal compound (if benzaldehyde reagent is used) or a blue to green color (if cinnamaldehyde reagent is used).

Can E. coli be indole negative?

Indole production is often used to differentiate E. coli from other indole-negative enteric bacteria because 96% of E coli are indole positive, whereas many enterobacterial species are negative in the indole reaction.

What is the positive result for indole test?

A positive result is shown by the presence of a red or red-violet color in the surface alcohol layer of the broth. A negative result appears yellow. A variable result can also occur, showing an orange color as a result.

Is Enterobacter indole positive or negative?

E. coli is indole-positive; Enterobacter aerogenes is indole- negative. Glucose is the major substrate oxidized by enteric bacteria for energy production.

What is the positive result for the indole test?

What bacteria are indole negative?

Bacteria which give negative results for the indole test include: Actinobacillus spp., Aeromonas salmonicida, Alcaligenes spp., most Bacillus spp., Bordtella spp., Enterobacter spp., most Haemophilus spp., most Klebsiella spp., Neisseria spp., Pasturella haemolytica, Pasturella ureae, Proteus mirabilis, Pseudomonas spp …

Is Shigella indole negative?

In the case of Shigella, the indole reaction is consistently negative only in specific serotypes within each traditional species of Shigella, including 7 of the 10 S. dysenteriae serotypes, 9 of the 15 S. flexneri serotypes, and S. sonnei (9).

Is Enterobacter aerogenes Gram negative?

Enterobacter is a genus of a common Gram-negative, facultative anaerobic, rod-shaped, non-spore-forming bacteria belonging to the family Enterobacteriaceae. Two of its well- known species, Enterobacter aerogenes and E.

Is Escherichia coli positive or negative for indole?

E. coli bacteria are among the few species of lactose (LAC)-positive, oxidase-negative, gram-negative rods that are indole positive. Due to the infrequent isolation of non- E. coli strains that are indole positive, the spot indole test has been used for the rapid, presumptive identification of E. coli.

What substrate is used for the indole test?

There are two media that are used for this test: Sulfide-Indole-Motility (SIM) medium and Tryptone broth medium. This program uses the Tryptone broth medium. SIM is a nutrient medium which allows the detection of three different traits in bacteria: it contains sulfates to serve as the substrate for detecting sulfide (H2S) production; abundant tryptophan as a substrate for indole production; and its content of 0.5% agar limits bacterial swimming, thereby allowing detection of motility.

Is E. coli indole positive?

Indole positive organisms: Most strains of E.coli, P. vulgaris, M. morganii and Providenica are indole positive. Point to remember: Indole test can also aid in species differentiation. Klebsiella species: Klebs‪iella oxytoca is indole positive whereas Klebsiella pneumoniae is indole negative.

How is indole produced?

Indoles are primarily produced within the human body as a byproduct of the degradation process of the amino acid, tryptophan. It is processed and usually found to accumulate in human feces, and at high concentrations, indoles possess a strong, unpleasant fecal odor.