Alternate of Coagulase test

                               Alternate of Coagulase test

 

Both coagulase tests detect the enzymes that convert fibrinogen into fibrin. Human, rabbit or sheep pooled plasma is used as substrate for both tests. Slide coagulase test is simpler and faster as compared to tube coagulase test. The plasma could be carrier of many human and animal pathogens like HIV, HBV, HCV etc                    

Improperly stored plasma can lead to false positive or negative results. Citrated plasma may be unsuitable for this test if contaminated with citrate utilizing bacteria.

 

DNA-ase test:

 

This test is used to help in the identification of S. aureus which produces deoxyribonuclease (DNA-ase) enzymes. The DNA-ase test is particularly useful when plasma is not available to perform a coagulase test or when the results of a coagulase test are difficult to interpret.

 

Principle:

Deoxyribonuclease hydrolyzes deoxyribonucleic acid (DNA).The test organism is cultured on a medium which contains DNA. After overnight incubation, the colonies are tested for DNA-ase production by flooding the plate with a weak hydrochloric acid solution. The acid precipitates unhydrolyzed DNA. DNA-ase-producing colonies are therefore surrounded by clear areas due to DNA hydrolysis.

 

Required:

— DNA-ase agar plate No. 33 Up to six organisms may be tested on the same plate.

— Hydrochloric acid Reagent No. 431 mol/1 (1 N)

 

Method:

1 Divide a DNA-ase plate into the required number of strips by marking the underside of the plate.

2 Using a sterile loop or swab, spot-inoculate the test and control organisms. Make sure each test area is labelled clearly.

3 Incubate the plate at 35–37 C overnight.

4 Cover the surface of the plate with 1mol/l hydrochloric acid solution. Tip off the excess acid.

5 Look for clearing around the colonies within 5minutes of adding the acid.

 

Results:

*Clearing around the colonies . . . . .  . . DNA-ase positive strain.

*No clearing around the colonies . . . . . . . . DNA-asenegative strain

 

Note: Some methicillin resistant S. aureus (MRSA)strains give a negative DNA-ase test. Some coagulase negative staphylococci are weakly positive. 

Also, S. pyogenes, Moraxella and Serratia speciesfrequently give a positive DNA-ase test.

 

Controls:

*Positive DNA-ase control: 

Staphylococcus aureus.

*Negative DNA-ase control:

Staphylococcus epidermidis.