Biochemical Test By: Mahmoud W El-Hindi 2013

General General MicrobiologyMicrobiologyLaboratoryLaboratory

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By: Mahmoud W El-Hindi 2

Coagulase is a protein enzyme having a prothrombin-like activity capable of converting fibrinogen to fibrin, which results in formation of a visible clot.

The coagulase test is intended for use in differentiation of Stapylococcus. Aureus “pathogenic” from other coagulase negative Staphylococcus sp “non-pathogenic “.


1. Bound Coagulase ("clumping factor"):

This factor is found on the surface of cell walls, and converts fibrinogen to fibrin directly, with no involvement of plasma factors, and is not inhibitedinhibited by antibodiesantibodies to free coagulase.

The slide coagulase test detects “bound coagulase “.


2- Free Coagulase (“clotting factor”):

This factor is extracellular and reacts with a substance in plasma “serum serum factorfactor” (also referred to as coagulase-reacting factor coagulase-reacting factor “CRF”).


Free coagulase reacts with CRF to form a coagulase-CRF complex.

This complex acts indirectly to convert fibrinogen to fibrin, forming forming a clota clot.

The tube coagulase test detects ‘bound’ and ‘free’ coagulase.


This test is a confirmatory test for the differentiation of the pathogenic Staphylococcus aureus from the non-pathogenic Staphylococcus epidermidis


Slide Test: 1- Place a drop of coagulase plasma

(rabbit plasma) on a clean, dry glass slide.

2- Place a drop of distilled water or saline next to the drop of plasma as your negative control.

3- With a loop, emulsify an amount of the isolated colony being tested in each drop, inoculating the water or saline first. Try to create a smooth suspension.


Tube test: 1. Place 0.5 mL of rabbit plasma in a

test tube. 2. Place 0.5 mL of distilled water in a

test tube. 3. Add one inoculating loopful of the

organism being tested to each tube.


Some bacteria may be able to use organic compounds other than sugars as their sole source of carbon.

The ability to metabolize citrate for

example is useful for differentiating among Enterobacteriaceae.


Simmons Citrate agar is a medium containing citrate as the sole carbon source and ammonium salts as the sole nitrogen source.

Organisms that metabolize citrate utilize the ammonium salts releasing ammonia and increasing the pH of the medium.


Brom thymol blue is present in the medium as the indicator dye.

It is green at neutral pH and deep blue above pH 7.6

Koser's citrate broth is another medium used to test for citrate utilization.

Growth is evidence of a positive reaction.By: Mahmoud W El-Hindi 22

POSITIVE CONTROL : Enterobacter aerogenes

Klebsilla

NEGATIVE CONTROL E. coli


1. Using a sterile inoculating needle, streak one organism over the surface of the agar slant, then stab the butt. Repeat with the second organism.

2. Incubate the tubes at 37 oC for 48 hours.

3. Examine for growth. Is there a change in the indicator dye in the agar?


Organisms that possess the enzyme, nitrate reductase reduces nitrate to nitrite.

Nitrite combines with an acidified substrate naphthylamine to form red colored end product.

If the organism has further reduced nitrite to nitrogen gas, the test for nitrate will yield negative (colorless) results.


An additional test for the presence of unreacted nitrate must be performed to validate such colorless results.

Metallic zinc catalyzes the reduction of nitrate to nitrite, thus, with the addition of zinc, a negative test will yield a red color, indicating the presence of unreacted nitrate.


Most Gram-negative bacilli are positive for nitrate reduction and negative for Nitrite reduction.

This test is used in the identification of only few gram negative bacilli such as Moraxella and Acinetobacter, some strains of Pseudomonas and Vibrio and Flavobacterium.


1- Inoculate a nitrate broth with the test organism.

2- Incubate at 37 C for 24 hours.3- Add 5 drops of reagent A

(Sulfanilic acid) and 5 drops of reagent B (naphthylamine) to the broth.


Positive: A red color develops in 1-2 minutes.

Negative: Colorless

NB: IF NEGATIVEAdd a pinch of zinc dust to the tubePositive: ColorlessNegative: Red color


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Biochemical Test By: Mahmoud W El-Hindi 2013

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