Streptococcus pyogenes: Morphology and Identification, Antigenic Structure - M Protein Flashcards
(11 cards)
Streptococcus pyogenes morphology and identification
Streptococcus pyogenes is a Gram-positive, cocci bacteria that appear in chains or pairs. It is beta-hemolytic, producing clear zones of hemolysis on blood agar. It is catalase-negative and facultative anaerobic. The bacteria are sensitive to bacitracin, which helps in identifying them from other streptococci. The species is classified as Lancefield group A based on the carbohydrate in its cell wall.
Antigenic structure of Streptococcus pyogenes - M protein
M protein is the major virulence factor in Streptococcus pyogenes. It is a surface protein that helps the bacterium evade phagocytosis by inhibiting the binding of complement factor C3b. M protein also plays a role in adherence to host cells and is responsible for the bacterium’s ability to cause both local and systemic infections.
Toxins and enzymes of Streptococcus pyogenes
1) Streptolysin O and Streptolysin S: These are hemolysins that contribute to the destruction of red blood cells and other tissue cells. 2) Pyrogenic exotoxins (Spe): These toxins are superantigens that induce a massive immune response, leading to conditions like scarlet fever and toxic shock syndrome. 3) Hyaluronidase: An enzyme that breaks down hyaluronic acid in connective tissue, aiding the spread of the bacteria. 4) DNAse: Enzyme that degrades DNA and helps bacteria evade the immune response.
Pathogenesis of Streptococcus pyogenes
Streptococcus pyogenes infects the host through mucosal surfaces, skin wounds, or through respiratory droplets. M protein helps the bacteria adhere to host cells and evade immune responses. The production of toxins and enzymes facilitates tissue invasion and inflammation. Streptococci can cause superficial infections (e.g., pharyngitis, impetigo) or invasive diseases (e.g., necrotizing fasciitis, toxic shock syndrome). The bacteria can also trigger poststreptococcal diseases such as rheumatic fever and glomerulonephritis.
Clinical findings of Streptococcus pyogenes infection
1) Pharyngitis (strep throat): Sore throat, fever, and swollen lymph nodes. 2) Scarlet fever: Characterized by a red, sandpaper-like rash, high fever, and strawberry tongue, typically following strep throat. 3) Impetigo: A highly contagious skin infection with honey-colored crusts. 4) Necrotizing fasciitis: Rapid tissue death, severe pain, and systemic toxicity. 5) Toxic shock syndrome: High fever, hypotension, and multiorgan failure due to superantigens.
Poststreptococcal diseases: Rheumatic fever
Rheumatic fever occurs as a complication following untreated or inadequately treated pharyngitis caused by S. pyogenes. It is an autoimmune disease where the immune system attacks the heart (rheumatic heart disease), joints (migratory arthritis), and skin (erythema marginatum). It can lead to permanent heart damage, especially valve involvement, and is most common in children aged 5-15 years.
Poststreptococcal diseases: Glomerulonephritis
Poststreptococcal glomerulonephritis is a kidney disease that can develop after a skin or throat infection with S. pyogenes. It is caused by immune complex deposition in the glomeruli, leading to inflammation. Symptoms include hematuria, edema, hypertension, and proteinuria. It is most commonly seen in children and can lead to renal impairment in severe cases.
Diagnostic laboratory tests for Streptococcus pyogenes
1) Rapid antigen test: Detects group A streptococcal antigens in throat swabs. 2) Culture: Blood agar shows beta-hemolytic colonies. Identification can be confirmed by bacitracin sensitivity. 3) ASO titer (Antistreptolysin O): Elevated levels indicate recent infection. 4) PCR: Used to identify bacterial DNA, especially in cases of invasive disease.
Treatment of Streptococcus pyogenes infection
Treatment of S. pyogenes infections typically involves antibiotics, primarily penicillin. In cases of penicillin allergy, alternatives like erythromycin or cephalosporins may be used. For severe infections like necrotizing fasciitis or toxic shock syndrome, broad-spectrum antibiotics and surgical intervention may be required. Early treatment of pharyngitis helps prevent complications like rheumatic fever.
Epidemiology of Streptococcus pyogenes
Streptococcus pyogenes is a common cause of human infections worldwide. It is transmitted via respiratory droplets, direct skin contact, or fomites. The bacteria typically colonize the throat, skin, or mucosal surfaces. Outbreaks are common in crowded settings like schools and daycare centers. Seasonal peaks occur during the winter and spring, correlating with respiratory viral infections.
Control and prevention of Streptococcus pyogenes infections
Prevention of S. pyogenes infections involves prompt treatment of pharyngitis with antibiotics to prevent complications like rheumatic fever. Good hygiene practices, such as handwashing and avoiding close contact with infected individuals, can help reduce transmission. There is no vaccine available for S. pyogenes, but prophylactic antibiotics are recommended for individuals at high risk of rheumatic fever or following certain types of surgery.
