Streptococcus Pyogenes

Streptococcus Pyogenes

Introduction

Streptococcus pyogenes (Group A streptococcus ) is a Gram-positive nonmotile , nonsporeforming coccus that occurs in chains or in pairs of cells . Individual cells are round-to-ovoid cocci , 0 .6-1 .0 micrometer in diameter (Figure 1 . Streptococci divide in one plane and thus occur in pairs or (especially in liquid media or clinical material ) in chains of varying lengths . The metabolism of S . pyogenes is fermentative the organism is a catalase-negative aerotolerant anaerobe (facultative anaerobe , and requires enriched medium containing blood in order to grow . Group A streptococci typically have

a capsule composed of hyaluronic acid and exhibit beta (clear ) hemolysis on blood agar Streptococcus pyogenes is one of the most frequent pathogens of humans It is estimated that between 5-15 of normal individuals harbor the bacterium , usually in the respiratory tract , without signs of disease As normal flora , S . pyogenes can infect when defenses are compromised or when the organisms are able to penetrate the constitutive defenses . When the bacteria are introduced or transmitted to vulnerable tissues , a variety of types of suppurative infections can occur

Reservoir of infection of Sreptococcus pyogenes

Human

Pathogenesis

Streptococci are members of the normal flora . Virulence factors of group A streptococci include (1 ) M protein and lipoteichoic acid for attachment (2 ) a hyaluronic acid capsule that inhibits phagocytosis (3 ) other extracellular products , such as pyrogenic (erythrogenic toxin , which causes the rash of scarlet fever and (4 ) streptokinase streptodornase (DNase B , and streptolysins . Some strains are nephritogenic . Immune-mediated sequelae do not reflect dissemination of bacteria . Nongroup A strains have no defined virulence factors

Summary of diseases caused by Streptococcus pyogenes

Suppurative conditions (active infections associated with pus ) occur in the throat , skin , and systemically

Throat

Streptococcal pharyngitis is acquired by inhaling aerosols emitted by infected individuals . The symptoms reflect the inflammatory events at the site of infection . A few (1-3 ) people develop rheumatic fever weeks after the infection has cleared

Skin

Impetigo involves the infection of epidermal layers of skin Pre-pubertal children are the most susceptible . Cellulitis occurs when the infection spreads subcutaneous tissues . Erysipelas is the infection of the dermis . About 5 of patients will develop more disseminated disease . Necrotizing fasciitis involves infection of the fascia and may proceed rapidly to underlying muscle

Systemic

Scarlet fever is caused by production of erythrogenic toxin by a few strains of the organism

Toxic shock is caused by a few strains that produce a toxic shock-like toxin

Non-suppurative Sequelae

Some of the antibodies produced during the above infections cross-react with certain host tissues . These can indirectly damage host tissues even after the organisms have beencleared , and cause non suppurative complications

Rheumatic fever . M protein cross reacts with sarcolemma . Antibodies cross-react with heart tissue , fix complement , and cause damage

Glomerulonephritis . Antigen-antibody complexes may be deposited in kidney , fix complement , and damage glomeruli . Only a few M-types are nephritogenic

Strep throat

Signs and Symptoms

The signs and symptoms of strep throat are red , sore throat with white patches on tonsils , swollen lymph nodes in neck , fever , and headache...