Microbiology
Summary
Streptococcus Pyogenes, also known as group A strep, is a gram-positive coccus that forms long chains or pairs. Group A strep is encapsulated and has a capsule made of hyaluronic acid. This bacteria is beta-hemolytic and can cause various infections. Some of the pyogenic infections include impetigo, which is a skin infection characterized by honey-crusted sores; pharyngitis or strep throat, characterized by an erythematous and inflamed throat; and cellulitis and erysipelas, which are skin infections causing erythema.
Infections caused by the Streptococcus pyogenes toxin, known as streptococcal pyrogenic exotoxin (SPE), can lead to conditions such as scarlet fever, which has symptoms like swollen strawberry tongue, pharyngitis, and a widespread rash; toxic shock syndrome (TSS); and the flesh-eating bacteria, necrotizing fasciitis. Two highly tested conditions associated with group A strep are rheumatic fever and poststreptococcal glomerulonephritis (PSGN). Both conditions are caused by the immune response to a Strep infection and can lead to severe complications if not promptly treated.
Group A strep can be differentiated from other types of Streptococcus based on its sensitivity to bacitracin. Finally, ASO antibodies are produced in response to streptolysin O, a virulence factor of group A strep that allows it to lyse red blood cells. ASO titers can be used to determine if an individual has recently had a group A strep infection.
Lesson Outline
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FAQs
Streptococcus pyogenes, also known as group A strep, causes various illnesses including strep throat, impetigo, cellulitis, necrotizing fasciitis, and toxic shock syndrome. Additionally, it can lead to serious nonsuppurative complications like acute rheumatic fever and poststreptococcal glomerulonephritis.
Streptolysin O and streptokinase are two important virulence factors that contribute to the pathogenesis of Strep pyogenes infections. Streptolysin O is a pore-forming toxin that damages host cells and tissues, which enables the bacteria to evade the immune system. Streptokinase is an enzyme that helps dissolve blood clots, allowing the bacteria to spread through the bloodstream and invade other tissues.
Bacitracin sensitivity is an important feature that distinguishes group A strep from other Streptococcus species. A simple laboratory test, known as the bacitracin test, can be performed, where the bacteria are streaked onto a blood agar plate with a bacitracin disk. If the bacteria are sensitive to bacitracin, a zone of inhibition will be observed around the disk, confirming the presence of group A strep like Streptococcus pyogenes.
The M protein is a major surface protein present on Strep pyogenes, which plays a crucial role in the bacterium's ability to cause disease. The M protein allows the bacteria to adhere to host cells, resist phagocytosis by the host immune system, and helps in the formation of biofilms. This enhances the ability of the bacteria to successfully colonize and invade host tissues, contributing to its virulence.
ASO (antistreptolysin O) antibodies are produced by the host immune system in response to the streptolysin O toxin released by Streptococcus pyogenes. The presence of elevated ASO antibody levels in the blood can indicate recent or active group A strep infection. Measuring ASO antibody titers is helpful in diagnosing poststreptococcal complications, such as acute rheumatic fever and poststreptococcal glomerulonephritis, as these titers usually peak 3 to 6 weeks after the initial infection.
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