Bacteriology

STREPTOCOCCUS PYOGENES

Written by MicroDok

Streptococcus pyogenes is a Group A Streptococcus species that is non-spore forming, Gram-positive, non-motile round to ovoid bacterium that occur in chains, singly (as coccus) or in pairs (as diplococci). It is the most common cause of pharyngitis (sore throat) in humans. Other streptococcal diseases caused by Streptococcus species in humans include scarlet fever, cellulitis, necrotizing fasciitis, erysipelas, impetigo, tonsillitis, pneumonia, otitis media, and septicaemia. S. pyogenes is a beta-haemolytic Streptococcus species that is catalase negative and facultatively anaerobic. It requires enriched media (e.g. agar supplemented with sheep blood) to grow effectively in the laboratory.

Streptococcus species are complex bacterial species classified into different groups based on their clinical significance and ability to haemolyse or breakdown blood. Some of the notable Streptococcus groups are Group A streptococci (e.g. S. pyogenes) which cause beta-haemolysis, Group B streptococci (e.g. S. agalactiae) which cause α, β, and γ haemolysis, Viridans streptococci group (which contain non-typable or groupable Streptococci species such as S. mutans), Group D streptococci (e.g. Enterococcus faecalis and S. bovis), and other untypable or ungroupable Streptococcus species including S. pneumoniae (pneumococcus) that has no specific antigenic substance for their grouping.

The groupings of Streptococcus species is based on serological data and/or specific antigenic structures associated with each of the streptococcal species, and this classification is based on the Lancefield classification – which classifies bacteria according to specific cell wall and capsular antigens found on the bacteria. E. faecalis is also known as S. faecalis. Lancefield classification (as described by the famous microbiologist Rebecca Lancefield) grouped Streptococcus species into groups A-O. The throat and skin are the main reservoir of S. pyogenes in the human body.     

PATHOGENESIS OF STREPTOCOCCUS PYOGENES INFECTION

S. pyogenes causes infections of the upper respiratory tract and other invasive infections of the skin and connective tissues of humans. Like Staphylococcus species, S. pyogenes produces a wide variety of virulence factors which spur the many diseases that they produce in humans. Some of the extracellular enzymes and toxins produced by S. pyogenes include streptolysin O and S, streptokinase, Dnase, hyaluronidase, M protein, haemolysins, and streptococcal pyrogenic exotoxins. Streptolysin O (SO) is an immunogenic, haemolytic and cytolytic toxin produced by S. pyogenes. It is antigenic in nature, thus stimulating the production of anti-streptolysin O (ASO) antibody in infected individuals; and can be used to diagnose recent S. pyogenes infections. SO is oxygen-labile (i.e. it is not active in the presence of oxygen), and is thus inactivated by oxidation.

Streptolysin S (SS) is a non-immunogenic, haemolytic, oxygen-stable toxin produced by S. pyogenes. SS is not antigenic, and thus forms no antibody in infected individuals. Both SO and SS destroy the cell membranes of red blood cells (RBCs) and other host cell membranes. Streptococcal pyrogenic exotoxins also known as erythrogenic toxin is mainly responsible for the rashes produced in scarlet fever infection; and the toxins (which are superantigens) produces similar clinical signs as that of toxic shock syndrome toxin (TSST) caused by S. aureus. M protein (fibronectin-binding protein) appears as hair-like structures on the cell wall of S. pyogenes; and it is primarily responsible for protecting the pathogen from host phagocytic action. S. pyogenes gains entry into the human body through mucous membranes or fractured skin; and the pathogen multiplies rapidly and evades some mechanisms of the host immune system (e.g. phagocytosis).

Upon entry, the pathogen causes a variety of acute, invasive and systemic infections such as impetigo (infection of the superficial layers of the skin), scarlet fever, a widespread skin rash, rheumatic fever (autoimmune disease involving the joints), necrotizing fasciitis (inflammation of skeletal muscles), pharyngitis (sore throat infection), tonsillitis (inflammation of the tonsils), cellulitis (infection of the deep layers of the skin), puerperal fever (sepsis after childbirth) and other severe invasive infections that may affect the heart and kidney and other vital organs of the body.

LABORATORY DIAGNOSIS OF STREPTOCOCCUS PYOGENES INFECTION

The laboratory diagnosis of S. pyogenes infections is based on the isolation and identification of the pathogen in culture and through microscopic and serological tests. Throat swab, wound swab, blood, pus and swabs from skin lesions are some clinically important specimens required for the laboratory diagnosis of S. pyogenes infections. Latex agglutination tests that detect ASO produced by S. pyogenes are commercially available for laboratory diagnosis of the disease. Gram stained smears of wound infections and skin lesions may prove diagnostic but stained smears of samples in pharyngitis are clinically irrelevant because Viridans Streptococci species are members of the human normal flora and may be indistinguishable from S. pyogenes in stained smears. Rapid diagnostic test kits (e.g. ELISA) are usually employed in the laboratory diagnosis of sore throat caused by S. pyogenes. S. pyogenes grow on blood agar and chocolate agar to produce small haemolytic colonies. S. pyogenes is susceptible to bacitracin but not optochin; and biochemically, it is catalase negative.

S. pyogenes grow on blood agar and chocolate agar to produce haemolytic colonies.

TREATMENT OF STREPTOCOCCUS PYOGENES INFECTION

Therapy for S. pyogenes infections is based on antimicrobial susceptibility test result and the type of prevailing disease. Penicillin G, erythromycin, cephalosporins and fluoroquinolones are some of the antibiotics of choice used to manage infections caused by S. pyogenes. Treatment of Streptococcal infections is important in order to prevent the spread of the pathogen to susceptible individuals and also to ease the already infected patient from the symptoms of the disease.  

PREVENTION AND CONTROL OF STREPTOCOCCUS PYOGENES INFECTION

No vaccine currently exists for immunization against diseases caused by S. pyogenes. Vaccines are only available for infections caused by S. pneumoniae. Prevention and control of S. pyogenes infection transmission is usually based on effective treatment of infected individuals. Infected individuals should be properly treated, and contact with non-infected people should be restricted as much as possible.

REFERENCES

Prescott L.M., Harley J.P and Klein D.A (2005). Microbiology. 6th ed. McGraw Hill Publishers, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.

Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA:  Lippincott-Raven Publishers. USA.

Basic laboratory procedures in clinical bacteriology. World Health Organization (WHO), 1991. Available from WHO publications, 1211 Geneva, 27-Switzerland.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 1. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 2. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R., Rosenthal K.S., Kobayashi G.S., Pfaller M. A. (2002). Medical Microbiology. 4th edition. Mosby Publishers, Chile.

 

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