Tests for Strep and Staph

Clinical Pathology
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اشترك في: الخميس إبريل 04, 2013 10:28 pm

Tests for Strep and Staph

مشاركةبواسطة دكتور كمال سيد » الأربعاء ديسمبر 11, 2013 9:28 pm

Tests for Strep and Staph

Streptococci and staphylococci are both gram-positive
spheres (cocci) and are responsible for a wide variety of
clinical diseases.
It is often necessary to differentiate
between these two organisms to prescribe the appropriate
The first way to differentiate them is to
examine their appearance on a Gram stain.

line up one after the other like a
strip of button candy,

while staphylococci appear as a cluster that can be visualized
as a cluster of hospital staff members posing for a group shot


in the figure above
Staphylococci lie in grape-like clusters as
seen on Gram stain.
Visualize this cluster of hospital
staff posing for a group photo.

Staphylococcus aureus is
catalase-positive, thus explaining the cats in the group
("Staphylococcus aureus (aureus means "gold
can be differentiated from the other beta-hemolytic
cocci by their elaboration of a golden pigment when cultured
on sheep blood agar.
Notice that our hospital
Staff (Staph) all proudly wear gold medals around their necks

A second method to differentiate streptococci
from staphylococci involves the enzyme
catalase. A

quick look at our
staff (Staph) picture reveals that a
CAT has joined them, so the
(staff picture is CAT(alase
That is, staphylococci possess the enzyme
catalase, whereas streptococci do not.
are thus referred to as catalase positive while streptococci
are catalase negative.

Catalase converts H2O2
(hydrogen peroxide, which is used by macrophages
and neutrophils) into 1120 and 0

2 . To test for catalase,

a wire loop is rubbed across a colony of gram-positive
cocci and mixed on a slide with 11202.
If bubbles appear,
the enzyme catalase must be present, and so staphylococci are present


from : Clinical Microbiology Made Ridiculously Simple

http://www.md4aa.com/vb/showthread.php? ... post116842

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