Hippurate Test
Hippurate hydrolysis
test is used to detect the ability of bacteria to hydrolyse substrate hippurate
into glycine and benzoic acid by action of hippuricase enzyme present in
bacteria. Hippuricase is a constitutive enzyme that hydrolyzes the substrate
hippurate to produce the amino acid glycine. Glycine is detected by oxidation
with Ninhydrin reagent, which results in the production of a deep purple color.
Objective
To detect the
production of the enzyme hippuricase for the presumptive identification of a
variety of microorganisms.
Principle
Hippurate is the
glycine conjugate of benzoic acid. When hippurate is hydrolysed by an
organism glycine and benzoic acid are formed. Glycine is deaminated by the
oxidizing agent ninhydrin, which is reduced during the process. The end
products of the ninhydrin oxidation react to form a purple-colored product.
Reagent and material
Method
- Hippurate disk
- Ninhydrin reagent
- Normal saline
- Blood culture media growth (18-24h)
- Culture tube 12x75 mm
- Forceps.
- loop
- Incubator.
Method
1. Add 200 mL of sterile water to a 12 ×75 mm test tube.
2. Make a heavy suspension(4-5 colonies) of the organism to be
tested.
3. Using heated forceps, place a rapid hippurate
disk in the mixture.
4. Cap and incubate the tube for 2 hours at35- 37°C;
use of a water bath is preferred.
5. Add 200 mL ninhydrin reagent and reincubate
for an additional 15 to 30 minutes.
6. Observe the solution for the development of a
deep purple color.
Expected
Results
- Positive: A positive test is indicated by the appearance of
a deep blue/violet color in 30 minutes.
- Negative: Colorless or slightly yellow pink color
Uses
- The hippurate test is most
frequently used in the identification of Gardnerella vaginalis,
Streptococcus agalactiae, Campylobacter jejuni, and Listeria
monocytogenes.
- Used in the differentiation of
β-hemolytic Streptococcus agalactiae from other β-hemolytic streptococci.
- Aids in the separation
of Campylobacter jejuni and Campylobacter
coli strains.
Limitations
- A false-positive result may
occur if incubation with ninhydrin exceeds 30 minutes.
- The test medium must contain
only hippurate, because ninhydrin might react with any free amino acids
present in growth media or other broths.
- Hippurate hydrolysis is also
found in some species of other genera, such as Bacillus,
Corneybacterium, Enterobacteriaceae, and others.
- The hippurate solution
deteriorates in 7 days at 4°C.
- The ninhydrin solution
deteriorates in 6 months.
- It is recommended that
biochemical, immunological, molecular, or mass spectrometry testing be
performed on colonies from pure culture for complete identification.
- Insufficient inoculum may result in erroneous results.
1-Positive : Streptococcus agalactiae
2-Negative : Streptococcus pyogenes
References
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