Question

1- What is the purpose of the IMViC series of tests? Explain the underlying biochemistry of each reaction and describe how each test is carried out. 100 marks

2-“The human microbiome is our newest organ.” Discuss this statement with references to the normal flora inhabiting different body sites and its associated benefits.100 marks

Answer 1

1.

• IMVic includes a series of tests which includes:

I = Indole

M= Methyl red (MR)

V=Voges Proskauer (VP)

I= filler word

C = Citrate

• These tests are used for identification of bacteria belonging to Enterobacteriaceae.

Indole:

• Indole test detects the ability of the organism to produce enzyme Tryptophanase.
• The organism that can produce enzyme tryptophanase, can cause deamination and hydrolysis of amino acid tryptophan, to generate Indole.
• Indole production is detected by adding Kovac’s reagent.
• Kovac’s reagent contain 4- Dimethyl amino benzaldehyde, which reacts with indole to produce red colored compound.
• Positive test is indicated by red colored ring formed on surface of media.

MRVP:

• MRVP test includes a medium or broth of same composition, which is split into two parts to perform MR using methyl red indicator (2- 5 drops) and VP test using Barritt’s reagent.
• A specific pure culture sample of culture may be inoculated in MRVP broth with inoculation loop.
• The medium contains glucose or dextrose, which is fermented by bacteria to produce positive result.
• MR test determines the ability of the organism to perform mixed acid fermentation.
• VP test determines the ability of the organism to perform 2,3 Butanediol fermentation.
• MRVP tubes after inoculation are incubated for 48-72 hours.

Citrate:

• Citrate test is used for bacterial identification as separate or as part of IMViC test and mostly used for differentiation of Enterobacteriaceae.
• The media used is Simon’s citrate.
• The media contain citrate as sole source of sugar.
• The organisms that can grow on the media, can utilize citrate by producing citrate permease. Also, Ammonium di-hydrogen phosphate in media is converted to ammonia and ammonium hydroxide. This results in an alkaline pH. The pH indicator Bromo-thymol blue turns blue from green.
• Thus, positive result is indicated by conversion of green color to blue color.