During microbiology practical you have been given with a wound swab for culture & identification. The swab has been inoculated on Blood agar and MacCondey agar. After 24 hour of incubation examiner allow you to use only blood agar plate. The organism is alpha-hemolytic and catalase negative and a gram stain indicate that, it is a Gram positive coccus.
Examiner’s viva questions:
a) What are the possible identities of the organism at this point?
b) What type of colonial morphology on MacConkey agar would you expect?
c) What are the tests need to be done to identity the organism?
Since the given organism is taken from wound swab, Staphylococci (Most common cause), Enterococci and Streptococci (Group A, B & D) were predicted; but the isolate is catalase negative. So Staphylococci were ruled out. Wound infections with viridans streptococci are very rare.
Among the streptococci Group A & B gives you beta hemolysis. So, Group D Streptococci and Enterococci are the possible identities of the organism at this point. Both of them gives you pin pointed lactose fermenting colonies on Macconkey agar. And both of them give positive reaction with Bile-Esculine test.
Tests to be done to differentiate Group D Streptococci and Enterococci:
- 6.5% NaCl Tolerance test (or)
- PYR test
+ comments + 2 comments
1) The possible identities of the organisms will be the following:
a) Streptococcus sp. or Enterococcus sp.the reasons being it is gram positive coccus, catalase negative and alpha hemolytic on BA.
b) The expected colony morphology on Mac Conkey agar would be either magenta coloured colonies or pale pink colonies with respect to Enterococcus sp. and Streptococcus sp. respactively.
c) `The Following tests are recommended:
i)PYR test - For Enterococci and Group A Streptococci.
ii)Ninhydrin test - For Group B streptococci.
iii)CAMP test - For Group B Streptococci.
iv) Pyruvate Fermentation Test - For Group D speciation and Enterococci speciation.
Based on the positive and negative results from the tests one by one, we will be able to identify the unknown organism.
Thanks to Anupama for the excellent discussion on this post.
Hats off to you. Keep it up.
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