Coagulase Test

Coagulase Test to identify Staphyloccus aureus.
Picture Credit: Ms Samjhana Kapali (PhD candidate in US)

The coagulase test is a biochemical test used to differentiate between two types of Staphylococcus bacteria: Staphylococcus aureus (coagulase-positive) and coagulase-negative Staphylococcus species. Here's how the test works:

Principle:

• Coagulase is an enzyme produced by some strains of Staphylococcus aureus. It acts on fibrinogen in the plasma to form fibrin, resulting in the formation of a clot.
• In the coagulase test, a plasma-based medium (usually rabbit plasma) is inoculated with the test organism. If the organism produces coagulase, it will cause the plasma to clot.

Procedure:

1. Inoculate a tube of rabbit plasma with a loopful of the test organism.
2. Incubate the tube at 37°C for up to 24 hours.
3. Examine the tube for the presence of clot formation.

Interpretation:

• Positive Result: Clot formation in the plasma indicates that the organism is coagulase-positive. This is characteristic of Staphylococcus aureus.
• Negative Result: No clot formation in the plasma indicates that the organism is coagulase-negative. This includes species such as Staphylococcus epidermidis, Staphylococcus saprophyticus, and others.

Precautions:

• Ensure that the plasma used is fresh and has not been subjected to repeated freeze-thaw cycles, as this can affect the test results.
• Incubate the inoculated tubes for the recommended time and temperature to allow sufficient time for clot formation.

Clinical Significance:

• Staphylococcus aureus is a common pathogen associated with various infections, including skin and soft tissue infections, pneumonia, and bacteremia.
• Coagulase-negative Staphylococcus species are often considered opportunistic pathogens and can cause infections, particularly in immunocompromised individuals or in medical device-related infections.

The coagulase test is a simple and rapid test used in microbiology laboratories to differentiate between Staphylococcus aureus and coagulase-negative Staphylococcus species, aiding in the diagnosis and management of bacterial infections.