Last updated on April 19, 2014 at 8:28 EDT

Streptococcus pyogenes

Streptococcus pyogenes is a spherical, Gram-positive bacterium is the cause of Group A streptococcal infections. It displays streptococcal group A antigen on its cell wall. When it is cultured on blood agar plates it produces large zones of beta-hemolysis. They are catalase-negative and in ideal conditions it has an incubation period of about 1-3 days. It is an infrequent part of the skin flora.

It is the cause of many important human diseases, ranging from mild superficial skin infections to life-threatening system disease. The skin and throat are typically where infections start. Pharyngitis and localized skin infections are examples of mild S. pyogenes infections. S. pyogenes invasion and multiplication in the fascia can lead to necrotizing fascitis, a potentially life-threatening condition requiring surgical treatment.

Some infections can be associated with the release of bacterial toxins due to certain strans of S. pyogenes. Some throat infections can lead to scarlet fever while other toxigenic S. pyogenes infections may lead to streptococcal toxic shock syndrome. It can also cause disease in the form of postinfectious syndromes. These autoimmune-mediated complications only follow a small percentage of infections. Rheumatic fever is characterized by inflammation of the joints and/or heart following an episode of streptococcal pharyngitis.

Group A streptococci infection has been proposed to be a risk factor in the development of obsessive-compulsive disorders and/or tic disorders in children. There is however, controversy over the reality of this disease since no studies have proved or disproved its existence.

The bacterium remains acutely sensitive to penicillin and failure of treatment with penicillin is attributed to other local commensal organisms. Some strains have developed resistance to macrolides, tetracyclines, and clindamycin.

S. pyogenes has several virulence factors enabling it to attach to host tissues, evade the immune response, and spread by penetrating host tissue layers. M protein also inhibits opsonization by the alternative complement pathway by binding to host complement regulators.

A throat swab is taken to the laboratory for testing. A gram stain is then performed to show Gram-positive cocci in chains. The organism is then cultured on blood agar with an added bacitracin antibiotic disk to show beta-haemolytic colonies and sensitivity for the antibiotic. A catalase test is administered which will show a negative reaction for all Streptococci. A serological identification of the organism involves testing for the presence of group A specific polysaccharide in the bacterium’s cell wall using the Phadebact test.

No vaccines are currently available to protect against S. pyogenes infection, but specific protective antibody has been shown to persist as long as 45 years after the original infection.

Streptococcus pyogenes