Question

What two laboratory tests would give you positive results to help you determine if your unknown bacterium is S. aureus? O MSA, catalase O MSA, coagulase O MSA, oxidase O coagulase, CAMP reaction O catalase, CAMP reaction

Answer 1

Mannitol Salt Agar (MSA) is a selective medium which contains all the nutrients required for the growth of micro-organisms.in addition to this they also contain 7.5% of sodium chloride which inhibits the growth of organisms other than Staphylococci. Mannitol is used as the fermentable Carbohydrate source whose fermentation can leads to acid production, and leads to the change in the colour of the indicator used i.e phenol red. Phenol red is red in colour at neutral pH while turns yellow in acidic medium. Staphylococcus aureus is a coagulase producer so ferments mannitol and produces acid which results in the production of yellow colonies. While the other species of Staphylococcus family which are coagulase negative produce red colonies. So this test can be used for the isolation and identification of Staphylococcus aureus.

the image shows the yellow colonies of Staphylococcus aureus.

Ho and Cew

Coagulase test: this test is utilized to differentiate the coagulase positive Staphylococcus aureus from the other coagulase negative Staphylococcus species. Coagulase is an enzyme which causes the plasma to clot by converting fibrinogen to fibrin. Staphylococcus aureus produces this enzyme in two form: free a(tube test ) and bound (slide test)

Bound coagulase is bound to the bacterial cell wall and reacts directly with fibrinogen which results in the direct clumping of the plasma on the slide when the Staphylococcus aureus cells are mixed.

Free coagulase involves the clumping of the plasma indirectly by the combination of the Staphylococcus cell with the coagulase-reacting factor (modified or derived thrombin complex molecule) which leads to the conversion of fibrogen to fibrin clot. Clot formation within 4 hours indicates a positive result.

Negative (no clumps) Positive clumps) Positive (clot) Negative (no clot) Slide Coagulase Test Tube Coagulase Test

Therefore, MSA and coagulase test are used for the identification of the Staphylococcus aureus bacteria.

So the correct option is MSA and coagulase.