Group A Beta Strep Incubation Time

Lonnie Medlock lonnie at
Fri Sep 19 23:11:56 EST 1997

What I have seen with selective media for group A strep is an increased time
to achieve good observable colonies with clear Beta hemolytic patterns. The
greater the selectivity of the medium, the longer the plate should be held
before calling the specimen negative. Typically with the TSA + 5% Sheep
Blood, that we manufacture, S. pyogenes 19615 will have good sized colonies
and beta hemolysis in 12 - 16 hours. TSA + BLD + SXT shows positive
reactions clearly in 18hrs while Beta SSA plates usually take at least 24
hours to achieve the same clarity and intensity. Some clinical strains
tested in parallel are so slow on this medium that it will take up to 36hrs
of incubation to see individual colonies which demonstrate true Beta
Hemolysis. I have tested our manufactured plates against all competitors
because I thought the reaction time was unacceptable, however all plates
purchased and paralleled gave equivalent reactions.
We also manufacture an enriched agar base with inhibitors that include
colistin and oxalinic acid. This medium grows all strains of Streptococci
(including pneumo) but will show good colonies and hemolytic reactions(12 -
16 hrs) comparable with a non selective blood agar plate.
So to make a long reply short, I guess it depends on the formulation of
selective medium you use. Remember there are always many compromises with
using selective media!

Stephanie Corbitt wrote:

> Lynn Gerber wrote:
> > There is a bit of controversy at our laboratory as to the length of
> > time a
> > culture for group A beta strep should be incubated.  We are using a
> > selective strep media which inhibits much of the normal flora.  The
> > controversy is whether the negative plates should be kept for more
> > than 24
> > hours.  In Bailey and Scott's Diagnostic Microbiology, 9th edition, it
> >
> > states that overnight incubation can be used for presumptive
> > identification.  No references are given.  Anyone out there have any
> > opinions, references, etc. regarding this topic?
> > Lynn Gerber

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