• Glucose – a monosaccharide • Lactose – a disaccahride

glucose + galactose

• Fermentation can produce acid and/or gas• A heterofermenter produces large amounts of

several acids• A homofermenter produces a small amount of a

single acid

• Phenol red becomes yellow in acid

CITRATE UTILIZATION• Catabolism of citrate produces sodium

carbonate, which is basic, so pH increases• The change in the pH is indicated by the dye

bromphenol blue in the medium, which changes from green to blue

• Also see growth of the cells• The citrate is brought into the cells by citrate

permease

UREA CATABOLISM

• Urea is broken down by urease to CO2 plus ammonia (NH3)

• Ammonia is basic, so raises the pH, which changes the phenol red to deep pink

H2S PRODUCTION AND DETECTION

• Produced from the amino acid cysteine, which contains sulfur

• H2S reacts with iron (Fe) to form FeS (iron sulfide), which is a black precipitate

• Also detects mobility of the bacteria, by spread of the black precipitate from the stab line

• Indole is produced by the catabolism of the amino acid tryptophan

• Addition of Kovac’s reagent produces a red color

INDOLE PRODUCTION

MR-VP TESTS• Methyl red – detects high level of acid production• Changes color at pH5• (Phenol red changes color at pH 6.8)

• V-P – detects the production of a particular alcohol (butanediol) that is made by Enterobacter. Addition of Barritt’s reagent produces a red color

• Both the acid and the alcohol are products of glucose fermentation

The IMViC series of tests is used to detect sewage contamination (for example of ocean water)

Indole Methyl Red V-P Citrate Escherichia coli

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Enterobacter aerogenes

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