Streptococcus
pyogenes is a gram positive cocci that causes group A streptococcal
infections. Strep pyogenes typically produces large zones of
beta-hemolysis and can be distinguished from other streptococcal
organisms because it is catalase negative and bacitracin sensitive. It
can cause a variety of disease including pyogenic infections of the
pharynx and skin. In the pharynx, it is the classic cause of bacterial
strep throat. In the skin, it can cause impetigo and cellulitis. This
bacteria also causes several diseases mediated by toxins including
scarlet fever and toxic shock like syndrome. Two of the major
complications of group A strep infections are mediated immunologic
mechanisms, including rheumatic fever caused by the development of
antibodies to M protein and acute glomerulonephritis.
Streptococcus pneumoniae is a Gram positive “lancet-shaped” diplococci
that is a major cause of pneumonia, meningitis, and otitis media.
Characteristically, this bacteria is alpha hemolytic, which causes dark
green colonies on blood agar. It is catalase dark green colonies on
blood agar, which allows it to be distinguished from catalase positive
Staph aureus. Strep pneumonia can be differentiated from streptococcus
viridans using an optochin test. Strep pneumoniae is optochin sensitive
while Strep viridans is optochin negative. The bacterium can also be
differentiated from Strep viridans because Streptococcus pneumoniae is
bile soluble while Strep viridans is not. This organism is encapsulated
with a positive quellung reaction. The capsule acts as a virulence
factor for the organism.
Streptococcus pneumoniae, , is a gram-positive diplococci bacteria. Classically, patients can complain of coughing up rusty sputum if they have pneumonia, , which can help with clinical diagnosis. Patients who have had splenectomies are at risk forsepsis,. Those who have sickle cell anemia,
especially children, are also at risk for sepsis, . Other
manifestations of strep pneumo can be remembered by the mnemonic MOPS,
This bacteria is also one of the most common causes of bacterial meningitis in adults, , and is the most common bacterial pathogen to cause otitis media, in the ear. Strep pneumonia is a very common cause of pneumonia, , and can also cause acute bacterial sinusitis,
Staphylococcus
epidermidis is a Gram positive cocci that is part of the human skin
flora. This organism is usually not pathogenic but can cause infection
in patients with compromised immune systems. Staphylococcus epidermidis
is also a large concern with patients with catheters or other prosthetic
devices because it is known to cause adherent biofilms that grow on the
devices. Characteristically, it is a catalase positive, coagulase
negative bacterium. Staphylococcus epidermidis can be differentiated
from staphylococcus saprophyticus, which is also catalase positive,
coagulase negative, but sensitivity to novobiocin. Because this organism
is a component of normal skin flora, it is a common contaminate of
blood cultures.
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