Yes, few numbers of Staph  aureus are coagulase negative and large numbers are coagulase positive.

yes.... some mutant species of stap. aureus are coagulase negative.

The majority of Staphylococcus aureus strains are coagulase-positive, whereas some may be atypical in that they do not produce coagulase. Besides, the performances of coagulase test (slide or tube methods) vary from setting to setting. For example, rapid plasma -due to its good clumping property- give more sensitive results. Besides, the specificity of sheep plasma is relatively low, and may not be appropriate for the tube coagulase test in some settings. Thus, where rabbit or horse plasmas are unavailable, sheep plasma may not replace. Human plasma gives discordant results since blood usually obtained from blood banks is outdated and may contain coagulase reacting factors and anti-staphylococcal antibodies. This kind of plasma is not recommended for coagulase tests. You should also check the accuracy of coagulase test with standard (positive and negative) strains of Staphylococcus.

There may be mutant strains of S. aureus that are coagulase negative (few) but you  may need to repeat your tube coagulase test using rabbit plasma (best). Perform DNAse test alongside.

Thank you for all information...i used rabbit plasma for my coagulase test..i also already check with standard staphylococcus species which is give clot formation..

Four hours incubation may be too short.  We routinely let the plasma incubate 24h at 37C (even though some suggest 4 hours at 37C and if negative, prolongate to 24h at room temperature).  The inoculum must be dense.

Yes, I also got some strains of coagulase negative S.aureus from clinical sources. But you have to amplify coa gene by PCR and also sequenced the 16 SrRNA to claim the strains really S.aureus. Because Coagulase negative staphylococci also seems some time as S.aureus when tested biochemically.

Hi there, we have a difference problem here. Some of our isolates of S. aureus are positive for nuc gene, in addition to forming clot on plasma (biochemically coagulase positive). However, we have some of these isolates that do not amplify the coa gene on PCR, even after having the nuc gene amplified. Has anyone experienced that?

Mr Uelinton

You need to use other primer for coa gene amplification, because of there are many primers couldn‘t amplify all coa alleles due to it high polymorphism gene

related question - are the negative coagulase Staphylocossus aureus strains as pathogenic as the coagulase positive? thanks in advance for your perspective

Some S.aureus strain gave negative coagulase.

Look back, do you culture sample on Bard Parker? if from food, you may select opac colonies for Coagulase, then most of them give possitive.

Therefore, i suggest if you could test the strain with API stap or other biochemical test to confirm.

Please aware that, not all coagulase positive staphylococci are Staphylococcus aureus.