unknown organism... advice on test...feedback

Question

Let's say I am doing a series of tests to identify an organism. I am doing VP test to differentiate S. aureus, saprophyticus, and epidermidis, and M. luteus. Then as expected, M. luteus is negative, the three staphs are all positive. To make this process faster AND more EFFICIENT, would it be more clever for the next step, to do MSA and then coagulase as needed,.... OR

...based off the colony morphology of my organism under the microscope (which is purple, "grape-like" clusters, so this would be coccus correct?) would I do coagulase first?

I do not want to assume my organism is S. aureus, but is it true that "grape-like clusters" fit the description aureus? 

If so based off of my descriptions and given info, please advise which is the better test to do after VP to have faster/more efficient identification of organism. 

Thank you.

Answer 1

Hello!

This is a great question; you want to not waste additional time doing unnecessary tests, but don't want to make any unnecessary assumptions that may lead to a false positive.

Firstly, it's probably a better idea to not characterize the organism based on morphology, since looks can be especially deceiving. Gram-stained S. aureus will group into 'grape like' clusters, however, most staphylococci will act the same way in presence of a gram stain. For this reason, relying on a chemical test is a much better option.

Since the coagulase test or the MSA test can both be used to differentiate S. aureus, the real question comes down to timing. The coagulase test for bound coagulase, or the 'slide coagulase test' takes roughly 30 seconds to incubate, whereas the free coagulase test, or the 'tube coagulase test' takes roughly 1 hour to incubate. In either case, these incubation times are quicker than the MSA test, which can take upwards of 12 hours before media begins to turn yellow (indicating S. aureus).

That being said, it may be more advantageous to do both. While the MSA plate is incubating, run the coagulase tests. If you get a distinctive positive, then you're all set. But if you have any difficulty interpreting your coagulase result, you can always go back to look at your MSA result.

Best of luck!