Group A strep

Group A Streptococcus

Group A Streptococcus (GAS), also known as Streptococcus pyogenes, is a species of Gram-positive bacteria that is responsible for a wide range of human diseases. These bacteria are known for causing mild infections such as pharyngitis (strep throat) and impetigo, as well as severe diseases like necrotizing fasciitis and streptococcal toxic shock syndrome.

Characteristics

Group A Streptococcus is a beta-hemolytic bacterium, meaning it completely lyses red blood cells when grown on blood agar plates, producing a clear zone around the colonies. It is a facultative anaerobe and is catalase-negative. The bacteria are typically arranged in chains and are non-motile.

Pathogenesis

GAS is a highly virulent pathogen due to its ability to produce a variety of virulence factors. These include:

• M protein: A major virulence factor that helps the bacteria evade phagocytosis by the host's immune system.
• Streptolysins O and S: Hemolysins that contribute to the lysis of host cells.
• Streptokinase: An enzyme that breaks down blood clots, facilitating the spread of the bacteria.
• Hyaluronidase: An enzyme that degrades connective tissue, aiding in tissue invasion.
• Exotoxins: Such as the pyrogenic exotoxins, which can lead to scarlet fever and toxic shock syndrome.

Diseases Caused

Group A Streptococcus is responsible for a variety of diseases, ranging from mild to life-threatening:

• Pharyngitis: Commonly known as strep throat, characterized by sore throat, fever, and swollen lymph nodes.
• Scarlet fever: A disease that can follow strep throat, marked by a red rash and high fever.
• Impetigo: A superficial skin infection, often seen in children.
• Cellulitis: A deeper skin infection that can spread rapidly.
• Necrotizing fasciitis: A severe, rapidly progressing infection that destroys tissues.
• Streptococcal toxic shock syndrome: A life-threatening condition characterized by shock and multi-organ failure.

Diagnosis

Diagnosis of GAS infections can be achieved through:

• Rapid antigen detection tests (RADTs): Used primarily for diagnosing strep throat.
• Throat culture: Considered the gold standard for diagnosing pharyngitis.
• Serological tests: Such as the anti-streptolysin O (ASO) titer, which can indicate a recent infection.

Treatment

GAS infections are typically treated with antibiotics. Penicillin remains the drug of choice for most GAS infections. In cases of penicillin allergy, erythromycin or other macrolides may be used. Severe infections like necrotizing fasciitis may require surgical intervention in addition to antibiotic therapy.

Prevention

Preventive measures include good hygiene practices, such as regular handwashing and avoiding close contact with infected individuals. There is currently no vaccine available for GAS.

Also see

• Streptococcus
• Beta-hemolytic streptococci
• Infectious diseases
• Antibiotic resistance