A critical procedural error can occur in some testing laboratories when diagnosing Staphylococcus aureus in conventional mastitis culture. This error occurs when the analyst relies solely on visual hemolytic changes of Staphylococcal colonies grown on Blood Agar to make the determination of Staphylococcus aureus or Staphylococcus spp. This diagnostic error may result in significant underreporting of Staph aureus positive cows, which consequentially may accelerate the rate of new infections or in extreme cases allow a major outbreak of Staph aureus to go undetected. Fortunately, the majority of strains of Staphylococcus aureus will produce the classic “beta zone” (β) around the colony, which is considered a diagnostic in bovine mastitis microbiology. However, what is significantly unrecognized in the industry is that a high percentage (frequently 30-40%) of Staph cows can harbor strains that produce only minor hemolysis or no hemolysis at all. This simple Coagulase Test media will allow detection of the coagulase enzyme by producing a partial or complete gelling of blood plasma, with overnight incubation or even as quickly as 4 hours.
The Udder Health Systems coagulase plasma is shipped in the liquid or frozen state and is ready to use. It is typically dispensed in approximately 1 ml aliquots to perform the test. Unlike lyophilized rabbit plasma, UHS coagulase plasma does not need to be reconstituted and is typically lower in test cost than rabbit plasma. It is good for several years in the frozen state and remains effective for at least 12 weeks at refrigerated temperatures.
Refer to table on back for expected test responses.
|E. Coli†||Excellent||False Positive|
All of these organisms will grow in the coagulase broth, this growth will produce a cloudy appearance. This is not to be interpreted as a positive reaction, only the coagulation reaction is a positive result. The coagulation can be detected in as little as 4 hrs. Typically the reaction is still readable in 24 hrs. Some false negatives will occur when coagulation occurs but the clot will lyse again by 24 hrs.
* All Staph spp. will grow in the coagulase broth, but only Staph aureus will generate a positive coagulation reaction. The coagulation can be detected in as little as 4 hrs. Typically the reaction is still readable in 24 hrs. Some false negatives will occur when coagulation occurs but the clot will lyse again by 24 hrs.
† Some strains of E. coli can produce the coagulase reaction. A false positive result can occur if the analyst mistakenly tests an E. coli colony. It is the responsibility of the analyst to only perform this test only on Staphylococcus genus isolates.
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