Streptococcus pyogenes, or beta streptococcus group A is a very common pathogenic bacterium in humans. Historically, Streptococcus pyogenes got its notoriety as the cause of puerperal fever, a life-threatening disease that was seen in women after childbirth, until Ignaz Semmelweis discovered that it could be prevented by physicians just washing their hands before each vaginal examination. Today, physicians treat infections caused by Streptococcus pyogenes on a daily basis.
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Morphology and Growth Requirements
Microscopically, Streptococcus pyogenes is a spherical bacterium (cocci) that appears in chain-like formations. It stains purple when the Gram stain method is used (gram-positive).
Macroscopically, when the bacteria are grown on agar that contains sheep blood, it will appear as a translucent colony with a zone of hemolysis surrounding the colony. This is due to specific enzymes produced by this type of streptococcus species.
S. pyogenes is considered a facultative anaerobic bacterium, which means it can grow in the presence or absence of oxygen. Growth is stimulated by incubating in an environment with increased carbon dioxide.
Streptococcus pyogenes produces numerous virulence factors that lend to its pathogenicity, or disease-causing capabilities.
It contains a capsule that helps allow the bacteria to disguise it from getting engulfed by white blood cells (phagocytosis). It also contains proteins on its cell wall that allows it to adhere to epithelial cells, which allows it to produce disease.
S. pyogenes also produces several enzymes that give the bacteria the ability to destroy tissue and spread. Lastly, this bacterium also produces a wide variety of toxins that can produce generally mild symptoms like a rash, to toxins that can cause multi-organ failure.
Probably the most common disease caused by Streptococcus pyogenes is pharyngitis, or strep throat. Strep throat is very common in school-aged children, particularly in the winter and spring months. Untreated strep throat can lead to more serious complications like rheumatic fever; however, this is relatively uncommon.
It is also a cause of several skin infections such as impetigo and cellulitis. Impetigo is a characterized by a crusty lesion frequently found on the mouth area. Cellulitis typically occurs after a wound or burn where the bacteria enters and spread though the skin and lower tissues.
More serious, potentially life-threatening infections caused by Streptococcus pyogenes include necrotizing fasciitis (commonly called flesh-eating bacteria) and toxic shock syndrome. In addition, Streptococcus pyogenes can cause scarlet fever, septicemia and pneumonia. The death of Muppets creator Jim Henson was a result of an infection with Streptococcus pyogenes.
Penicillin is still the drug of choice for treating Streptococcus pyogenes infections. In cases when a person is penicillin allergic, erythromycin is an alternative treatment.