Differential media

Tue Mar 10 07:35:49 EST 1998

Koneman's Color Atlas of Diagnostic Microbiology has excellant 
pictures.  Coliforms such as E. coli should grow as bright red 
(purple?) colonies due to lactose fermentation.  Non-coliforms will 
produce no change in the color of the media.

MSA media turns yellow in response to mannitol fermentation, which is 
characteristic of S. aureus.  However, it is not correct to call S. 
aureus a pathogen, since it is a part of the normal flora in many 
healthy individuals.  it is an opportunistic pathogen.

Many bacteria will produe beta hemolysis on blood agar.  It is 
important to remember that a gram stain will not confirm whether you 
are looking at a staph or a strep.  Strep often don't chain out on 
solid media.  Pick isolated colonies and grow them in TS broth, or BHI 
or TM media.  Then perfrom a catalase test.  Strep are catalase 
negative, staph are catalase positive.  It is difficult to do the 
catalast from blood agar, since hemoglobin will give a false positive.

Jay Mone'

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