Download Cycle 33 Organism 4 - Streptococcus pyogenes

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MICROBIOLOGY LEGEND
CYCLE 33 – ORGANISM 4
STREPTOCOCCUS PYOGENES
Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A
Streptococcal infections. S. pyogenes displays Streptococcal group A antigen on its cell wall. S.
pyogenes typically produces large zones of beta-hemolysis when cultured on blood agar plates, and
are therefore also called Group A (beta-hemolytic) Streptococcus (GABHS). Streptococci are
catalase-negative. In ideal conditions, S. pyogenes has an incubation period of approximately 1–3
days.
Gram stain of Streptococcus pyogenes
Colonies of Streptococcus pyogenes
on blood agar exhibiting beta (clear) hemolysis Streptococcus pyogenes is one of the most frequent pathogens of humans. It is estimated that
between 5-15% of normal individuals harbour the bacterium, usually in the respiratory tract,
without signs of disease. As normal flora, S. pyogenes can infect when defences are compromised
or when the organisms are able to penetrate the constitutive defences. When the bacteria are
introduced or transmitted to vulnerable tissues, a variety of types of suppurative infections can
occur.
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Classification of Streptococci
Hemolysis on blood agar
The type of haemolytic reaction displayed on blood agar has long been used to classify the
streptococci. Beta-haemolysis is associated with complete lysis of red cells surrounding the colony,
whereas alpha-haemolysis is a partial or "green" haemolysis associated with reduction of red cell
haemoglobin. Non haemolytic colonies have been termed gamma-haemolytic. Haemolysis is affected
by the species and age of red cells, as well as by other properties of the base medium. Group A
Streptococci are nearly always beta-haemolytic; related Group B can manifest alpha, beta or
gamma haemolysis. Most of the oral Streptococci and Enterococci are non haemolytic. The
property of haemolysis is not very reliable for the absolute identification of Streptococci, but it is
widely used in rapid screens for identification of S. pyogenes.
Antigenic types
The cell surface structure of Group A Streptococci is among the most studied of any bacteria. The
cell wall is composed of repeating units of N-acetylglucosamine and N-acetylmuramic acid, the
standard peptidoglycan. Historically, the definitive identification of Streptococci has rested on
the serologic reactivity of "cell wall" polysaccharide antigens as originally described by Rebecca
Lancefield. Eighteen group-specific antigens (Lancefield groups) were established. The Group A
polysaccharide is a polymer of N-acetylglucosamine and rhamnose. Some group antigens are shared
by more than one species. This polysaccharide is also called the C substance or group carbohydrate
antigen.
Diagnosis
Usually, a throat swab is taken to the laboratory for testing. A Gram stain is performed to show
Gram-positive cocci in chains. Then, the organism is cultured on blood agar with an added bacitracin
antibiotic disk to show beta-hemolytic colonies and sensitivity (zone of inhibition around the disk)
for the antibiotic. It is then cultured on non-blood containing agar; the catalase test is performed
which should show a negative reaction for all Streptococci. S. pyogenes is CAMP and hippurate
tests negative. Serological identification of the organism involves testing for the presence of
group A specific polysaccharide in the bacterium's cell wall using the Phadebact test.
Pathogenesis
Streptococcus pyogenes owes its major success as a pathogen to its ability to colonize and rapidly
multiply and spread in its host while evading phagocytosis and confusing the immune system.
Acute diseases associated with Streptococcus pyogenes occur chiefly in the respiratory tract,
bloodstream, or the skin. Streptococcal disease is most often a respiratory infection (pharyngitis
or tonsillitis) or a skin infection (pyoderma). Some strains of streptococci, show a predilection for
the respiratory tract; others for the skin. Generally, streptococcal isolates from the pharynx and
respiratory tract do not cause skin infections.
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S. pyogenes is the leading cause of uncomplicated bacterial pharyngitis and tonsillitis commonly
referred to as strep throat. Other respiratory infections include sinusitis, otitis, and pneumonia.
Infections of the skin can be superficial (impetigo) or deep (cellulitis). Invasive streptococci cause
joint or bone infections, destructive wound infections (necrotizing fasciitis) and myositis,
meningitis and endocarditis. Two post streptococcal sequelae, rheumatic fever and
glomerulonephritis, may follow streptococcal disease, and occur in 1-3% of untreated infections.
These conditions and their pathology are not attributable to dissemination of bacteria, but to
aberrant immunological reactions to Group A Streptococcal antigens. Scarlet fever and
streptococcal toxic shock syndrome are systemic responses to circulating bacterial toxins.
Treatment and prevention
Penicillin is still uniformly effective in treatment of Group A Streptococcal disease. It is important
to identify and treat Group A Streptococcal infections in order to prevent sequelae. No effective
vaccine has been produced, but specific M-protein vaccines are being tested.
Frequency
Infections with group A Streptococcus are observed worldwide. Prevalence of Streptococcal
pyoderma is higher in regions near the tropics. Aside from this observation, no geographic barriers
to infection with this ubiquitous organism are recognized.
Rheumatic fever is most frequently observed in the age group most susceptible to group A
Streptococcal infections (i.e., children aged 5-15 y). The attack rate following upper respiratory
tract infection is approximately 3% in individuals with untreated or inadequately treated infection.
Mortality/Morbidity
Infection with group A Streptococci leads to various clinical manifestations responsible for
considerable morbidity and, with increasing frequency, mortality. In addition, infection with this
organism leads to postsuppurative sequelae, particularly acute rheumatic fever and post
streptococcal glomerulonephritis (PSGN).
References
1. Todar’s Online Textbook of Bacteriology – Kennet Todar PHD
2. EMedicine Paediatrics – Mark R Schleiss
3. http://en.wikipedia.org/wiki/Streptococcus_pyogenes
Questions
1. What are the morphological characteristics of Streptococcus pyogenes?
2. Discuss the classification and pathogenesis of Streptococcus pyogenes.
3. What is the recommended treatment for Streptococcus pyogenes?
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