S. pyogenes is one of the most clinically important gram positive cocci. This species belongs to the Lancefield group A streptococci.
It is the most prevalent human bacterial pathogens.
It is associated exclusively with infections related to the humans.
It causes wide range of suppurative (pus producing) infections in the respiratory tract and skin, like tonsillitis, pharyngitis and cellulitis, life-threatening soft tissue infections, and certain types of toxin-associated reactions.
Furthermore, some of these infections may cause severe nonsuppurative diseases due to immunological reactions induced by the S. pyogenes.
They are post infection sequelae including rheumatic fever and acute glomerulonephritis.
In addition it causes peritonsillar abscess (quinsy), scarlet fever, otitis media, cellulitis, impetigo, necrotizing fasciitis, erysipelas, puerperal sepsis, septicaemia, and occasionally toxic shock syndrome.
Especially in colder months nasopharyngeal carriage is common in children.
S. pyogenes does not survive well in the environment.
Its habitat is infected patients and on skin and mucous membrane of normal human carriers.
S. pyogenes is usually spread person to person by skin contact and via the respiratory droplets.
There are many virulence factors like Streptolysins, Streptokinase, Hyaluronidase, Leukocidin, Lipoteichoic acid, M-proteins, nicotinamide adenine dinucleotidase, deoxyribonuclease and Erythrogenic toxin which are important for their infectivity.
Laboratory features and diagnosis
For the diagnosis of S. pyogenes, specimens are throat swabs, swabs of pus and serous fluid depending on the site of infection, blood for culture.
S. pyogenes is a gram positive cocci in chains. Normally found as short chains, but also in pairs and singly.
In fluid cultures long chains are formed. The organisms are non-motile and some strains are capsuled.
It is catalase negative.
Colony Morphology
Blood agar
pyogenes shows β-hemolytic colonies on blood agar.
This organism shows a complete haemolysis.
Usually colonies are small and measuring 0.5-1 mm in diameter.
Colonies are colourless or creamy colour, shiny and mucoid.
Haemolysis is more marked under anaerobic conditions.
For isolating S. pyogenes from patients with impetigo Crystal violet 1 in 50000 in blood agar is a used ,it is an inexpensive selective medium.
Crystal violet will inhibit the growth of S. aureus and enhance the growth of S. pyogenes.
MacConkey agar
No growth on MacConkey agar
Chocolate agar
Shows cream colour colonies on chocolate agar. Colonies are small and mucoid.
Moreover, S. pyogenes belongs to Lancefield Group A.
In order to identify the A antigen extracted from the cell wall of the bacteria (confirmed catalase negative, Gram positive streptococci) beta haemolytic colonies are grouped using group A antiserum.
Most laboratories use a rapid agglutination test.
S. pyogenes is Bacitracin sensitive (0.04 IU).
Performing a PYR test is a further way of establishing a presumptive diagnosis of S.pyogenes when the reagents for serogrouping are not available.
S. pyogenes is PYR positive. Additionally, S. pyogenes is always sensitive to benzylpenicillin and therefore placing a 1 μg disc of the antibiotic on a primary culture plate (well area) can also help to presumptively identify S.pyogenes.
Testing for ASO (anti-streptolysin O) antibody in serum is helpful in diagnosing rheumatic fever.
Following an infection, a high titre of ASO antibody can be investigated in most patients. For the investigation of post-streptococcal diseases, measurement of ASO antibody titre is important.
ASO titre is useful to identify rheumatic fever which usually develops 2–3 weeks or more after streptococcal sore throat, when it is often not possible to isolate S. pyogenes in culture.
Measurement of DNA-ase B (deoxyribonuclease B) antibody titre is useful for the investigation of acute glomerulonephritis occurred after skin infection by S. pyogenes
Treatment
Penicillin is the most effective drug for the treatment of S. pyogenes.
Erythromycin, Clarithromycin or Azithromycin is usually used to treat patients hypersensitive to penicillin.
Tetracycline also useful for the treatment.
For the treatment of necrotizing fasciitis and in streptococcal toxic shock syndrome Penicillin G plus Clindamycin are used.
Other important features
Rheumatic fever is prevented by rapid elimination of the infecting organism.
Prolonged prophylactic antibiotic therapy is done after an episode of rheumatic fever to prevent recurrent infection.
There are no vaccines against S. pyogenes.
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