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