Week 2 Laboratory: Blood Cultures

Hello everyone, it is me again.  Today in the laboratory we read the plates prepared yesterday and completed additional testing in order to come up with a final ID for our blood culture sample.  I am going to discuss the process I went through to identify the pathogen in my blood culture specimen.  The purpose of this discussion is to show how easily one pathogen can be mistaken for another and the importance of complete and thorough testing on a specimen.

I began by making a gram stain of my specimen using the blood from the blood culture bottle.  After allowing the stained slide to dry, I examined the slide under oil immersion.  I quickly identified the morphology of the bacteria as gram positive cocci in chains.  Having determined the morphology and gram stain, I then plated the specimen on the appropriate media, in this case SBA with a “P” disk.  I then incubated the plate overnight in CO2.  

The next day I retrieved my specimen and examined the macroscopic morphology of the bacterial colonies.  The colonies were small, white to gray in color, convex and semi-opaque.  The specimen also displayed beta-hemolysis.  The gram stain and colony morphology lead me to declare the preliminary ID to be beta-hemolytic Streptococcus.  I believed my specimen specifically to be S. pyogenes; the macroscopic and microscopic morphology fit that of S. pyogenes perfectly.  I performed two additional tests in order to come up with a final ID: a catalase test and a PathoDX Strep grouping testing.  As expected the catalase was negative and I believed I saw agglutination for Group A strep on the PathoDX card.  This supported my ID of S. pyogenes.  However, Mrs. Jeff told me this was incorrect.  I performed the PathoDX test again, this time inoculating the reagent broth with more colonies from my plate.  This time there was clear visual agglutination for Group G Strep, making my final ID Streptococcus dysgalactiae subsp. equisimilis, a Group C or G strep species.  The colony morphology for Group C and G strep is almost identical to that of Group A strep, making it fairly easy to confuse the two as I originally did.  So the lesson here is to always complete all necessary testing, never make a final ID on the colony morphology alone!

Which is which???  Pretty similar huh?  The specimen below is  Group G  Strep and the specimen above is Group A Strep.