Fish Microbiology and Quality Assurance

6. Coagulase test

This test is done to confirm the identification of enterotoxin producing virulent strains of Staphylococcus aureus. Rabbit plasma, which contains coagulase reacting factor (CRF is made to react with bacteria producing coagulase enzyme which results in clotting or coagulation of blood plasma. Any degree of coagulation, from a loose clot suspended in plasma to a solid immovable clot, is considered as positive result.
Coagulase test can be done by two methods: Slide method and Tube method.

A. Slide method: Mark a slide into two sections. Place a drop of sterile saline in both the section and emulsify a loopful of 18 – 24 h old culture with saline placed in one of section, and treat the other section as control. Add one or two drops of rabbit plasma to both the sections and mix well for 5 seconds. If it coagulates, the test is positive. Slide method is mainly used for detecting the presence of free coagulase.

B. Tube method: To a test tube containing 0.5 ml of rabbit plasma add 0.5 ml of 18 – 24 h old culture, incubate at 37⁰C and examine for clump formation in 2, 3, 6 and 24 h. If clump formation occurs, the test is considered as positive. A control to be maintained without adding bacterial culture. This method is mainly used for detecting the presence of bound coagulase.