Group A Beta Strep Incubation Time

Jim Mau elgusano at
Sat Sep 20 11:37:16 EST 1997

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
> Dear Lynn,
>     Group A Strep is easily identifiable on blood agar plates.  After an
> incubation period of 18-24 hours,  small pinpoint colonies that are beta
> hemolytic (complete hemolysis of RBCs & hemoglobin) can be easily
> observed.  I am not sure what type of selective strep media you are
> using, but if Strep is suppose to utilize some ingredient of your
> selective media and grow within an overnight (24 hour) incubation and
> you are unable to identify colonies after the 24 hours then the cultures
> should be considered negative for Group A Strep and no additional
> incubation period should be considered.    However, something could be
> wrong with your media.  If you are not culturing your Strep samples on
> blood agar,  maybe you should consider using both types of media  to see
> if there is some kind of problem with your selective media or if it is
> just that your samples do not contain Group A Strep.
> Steph

	The current issue of 'Clinical Infectious Diseases' (CID 1997;24 Sept.
p.574-583), Journal of the Infectious Diseases Society of America, has
published a set of practice guidelines for the diagnosis and management
of Group A Streptococcal Pharyngitis.  It states: "Throat swab specimens
should be obtained from the surface of both tonsils (or tonsillar
fossae) and the posterior pharyngeal wall.  Other areas of the
oropharynx and mouth are not acceptable sites for sampling,..."  The
guideline then goes on to state: "Once plated, cultures should be
incubated at 35 -37C for 18-24 hours before they are read.  However,
additional overnight incubation at room temperature allows
identification of a considerable number of positive throat cultures that
would not otherwise be identiified.  Thus, while initial therapeutic
decisions may be made on the basis of the results of an overnight
culture, it is advisable to reexamine plates at 48 hours that are
negative at 24 hours ..."  Reference cited is: Kellogg JA. Suitability
of throat culture procedures for detection of group A streptocci and as
reference standards for evaluation of streptococcal antigen detection
kits. J Clin Microbil 1990; 28:165-9.

Q. Are you collecting your specimen as discribed above?  If you are not
collecting your specimen correctly (say, as discribed above), and are,
for example, just touching the back of the throat, you will greatly
reduce the sensitivity of your methodology and potentially, the
reporting out of a false negative. Be sure to follow explicitly the
manufacturers directions on collection and methodology. These are the
conditions under which the product was developed and approved (read
'Cleared') for the indicated use. Not all products are necessarily the

James L. Mau, Biologist					Phone:	619-435-6904
Industry Consultant
Infectious & Parasitic Diseases
BME International					Fax:	619-435-7792
431 Country Club Lane
Coronado, CA  92118

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