|Rabbit plasma after centrifugation and separation
from red cells
|Coagulase test (Stahpylococcus aureus)
both tubes are positive
Coagulase is an enzyme produced by some bacteria that reacts with prothrombin in the blood, forming a complex named
staphylothrombin, which causes blood to clot by converting fibrinogen to fibrin. This mechanism protect bacteria from phagocytosis.
Frequently coagulase test is used to distinguish between different types of Staphylococcus isolates.
Harvest blood from a rabbit using EDTA to prevent coagulation. Separate plasma
from blood cells deposit by centrifugation. This should be carried out for a minimum
of 10 minutes at 1500-2000 rpm at room temperature. Do not use brake to stop
centrifuge. It will result two layers: plasma (up), erythrocytes (bottom).
Inocculate rabbit plasma with bacteria culture and incubate the tube in a 37 °C
incubataor for 3-4 hours. Examine hourly the clot formation. If negative, then
continue incubation up to 24 hours.
Result is positive if the serum coagulates resulting a clot (or a solid block when the
reaction is very intense). Handle the tube with care. The clot may be fragile and may
be disrupted before reading the reaction.
Result is negative if the plasma remains liquid.
Utilization of 2% Na citrate for coagulase tesr is no longer recommended. Some
bacteria can utilize citrate from plasma and false positive reactions may appear. In
order to avoid false positive coagulase reactions, tests should be performed with
plasmas which contain anticoagulants other than citrate, e.g. oxalate or heparin.
1. Helgomar Raducanescu, Valeria Bica-Popii,1986. Bacteriologie veterinara, Ed.
2. Margaret Barnett, 1992. Microbiology Laboratory Exercises. Wm. C. Brown Publishers.
3. Mushin, R. & Kerr, V. J. (1954). Clotting of Citrated Plasma and Citrate Utilization by Intestinal Gram-negative Bacilli. J. gen.
Microbiol. 10, 445-451.
(c) Costin Stoica