Streptococcus pyogenes

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Streptococcus pyogenes
S. pyogenes bacteria @ 900x magnification.
S. pyogenes bacteria @ 900x magnification.
Scientific classification
Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus
Species: S. pyogenes
Binomial name
Streptococcus pyogenes
Rosenbach 1884

Streptococcus pyogenes is a Gram-positive cocci that grows in long chains depending on the culture method.[1] S. pyogenes displays group A antigen on its cell wall and beta-hemolysis when cultured on blood agar plate. S. pyogenes typically produces large zones of beta-hemolysis, the complete disruption of erythrocytes and the release of hemoglobin, and it is therefore called Group A (beta-hemolytic) Streptococcus (abbreviated GAS).

Contents

[edit] Serotyping

In 1928, Rebecca Lancefield published a method for serotyping S. pyogenes based on its M protein, a virulence factor that is displayed on its surface.[2] Later in 1946, Lancefield described the serologic classification of S. pyogenes isolates based on their surface T antigen.[3] Four of the 20 T antigens have been revealed to be pili, which are used by bacteria to attach to host cells.[4] Currently, over 100 M serotypes and approximately 20 T serotypes are known.

[edit] Virulence factors

S. pyogenes has several virulence factors.[5] A carbohydrate capsule surrounds the bacterium, protecting it from attack by macrophages (part of the immune system). Further, there are proteins, lipoteichoic acids, embedded within the capsule (M protein) that also increase virulence by facilitating attachment to host cells.[6] M protein inhibits a branch of the immune system called the complement system, which binds to and destroys invading bacteria. However, the M protein is also the weakest point in this organism's defense as this is what antibodies produced by the immune system use to recognize the bacterium. M proteins are unique to each strain and identification can be used clinically to confirm the strain causing an infection.

S. pyogenes releases a number of proteins, including several virulence factors, into its host:

Streptolysin O and S
Toxins which are the basis of the organism's beta-hemolytic property. Streptolysin O is a potent cell poison affecting many types of cell including neutrophils, platelets, and sub-cellular organelles. It causes an immune response and detection of antibodies to it; antistreptolysin O (ASO) can be clinically used to confirm a recent infection.
Pyrogenic toxin
Found in strains of S. pyogenes responsible for scarlet fever.
Streptokinase
Enzymatically activates plasminogen, a proteolytic enzyme, into plasmin which in turn digests fibrin and other proteins.
Hyaluronidase
It is widely assumed that hyaluronidase facillitates the spread of the bacteria through tissues by breaking down hyaluronic acid, an important component of connective tissue. However, very few isolates of S. pyogenes are capable of secreting active hyaluronidase due to mutations in the gene that encode the enzyme. Moreover, the few isolates that are capable of secreting hyaluronidase do not appear to need it to spread through tissues or to cause skin lesions.[7] Thus, the true role of hyaluronidase in pathogenesis, if any, remains unknown.
Streptodornase
Most strains of S. pyogenes secrete up to four different DNases, which are sometimes called streptodornase. The DNases protect the bacteria from being trapped in neutrophil extracellular traps (NETs) by digesting the NET's web of DNA, to which are bound neutrophil serine proteases that can kill the bacteria.[8]
C5a peptidase
C5a peptidase cleaves a potent neutrophil chemotaxin called C5a, which is produced by the complement system.[9] C5a peptidase is necessary to minimize the influx of neutrophils early in infection as the bacteria are attempting to colonize the host's tissue.[10].
Streptococcal chemokine protease
The affected tissue of patients with severe cases of necrotizing fasciitis are devoid of neutrophils.[11]. The serine protease ScpC, which is released by S. pyogenes, is responsible for preventing the migration of neutrophils to the spreading infection.[12] ScpC degrades the chemokine IL-8, which would otherwise attract neutrophils to the site of infection. C5a peptidase, although required to degrade the neutrophil chemotaxin C5a in the early stages of infection, is not required for S. pyogenes to prevent the influx of neutrophils as the bacteria spread through the fascia.[10][12]
Other toxins
Including F-protein, Lipoteichoic acid and others.

[edit] Pathogenesis

Main article: Group A streptococcal infection

S. pyogenes is the cause of many important human diseases ranging from mild superficial skin infections to life-threatening systemic diseases. Infections typically begin in the throat or skin. Examples of mild S. pyogenes infections include pharyngitis ("strep throat") and a localized skin infection called impetigo. The diseases erysipelas and cellulitis are characterized by lateral spread and multiplication of S. pyogenes in deeper layers of the skin. S. pyogenes invasion and multiplication in the fascia result in the life-threatening disease necrotizing fasciitis.

S. pyogenes infections are sometimes accompanied by the release of toxins from the bacteria. Throat infections associated with toxin release lead to scarlet fever. Other toxigenic S. pyogenes infections may lead to streptococcal toxic shock syndrome, which can be life-threatening.

Poststreptococcal complications occur in a small percentage of infections and include the autoimmune conditions rheumatic fever and acute glomerulonephritis, which result from similarities between S. pyogenes and human tissue components. Both conditions appear several weeks following the initial streptococcal infection. Rheumatic fever can cause a type of arthritis, which recurs following each episode of streptococcal pharyngitis. Acute glomerulonephritis, which causes inflammation of the renal glomerulus, can follow pharyngitis or skin infection by S. pyogenes.

This bacterium is very sensitive to penicillin. Isolates resistant to penicillin or other β-lactams have not been discovered to date. Treatment failure with penicillin in the setting of streptococcal pharyngitis is attributed to other local commensal organisms producing β-lactamase or failure of adequate local tissue levels in the pharynx. Isolates occasionally reveal resistance to macrolides, tetracycline and clindamycin.

[edit] References

  1. ^ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill. ISBN 0-8385-8529-9. 
  2. ^ Lancefield RC (1928). "The antigenic complex of Streptococcus hemolyticus". J Exp Med 47: 9–10. 
  3. ^ Lancefield RC, Dole VP (1946). "The properties of T antigen extracted from group A hemolytic streptococci". J Exp Med 84: 449–71. 
  4. ^ Mora M, Bensi G, Capo S, Falugi F, Zingaretti C, Manetti A, Maggi T, Taddei A, Grandi G, Telford J (2005). "Group A Streptococcus produce pilus-like structures containing protective antigens and Lancefield T antigens". Proc Natl Acad Sci U S A 102 (43): 15641-6. PMID 16223875. 
  5. ^ Patterson MJ (1996). Streptococcus. In: Baron's Medical Microbiology (Baron S et al, eds.), 4th ed., Univ of Texas Medical Branch. (via NCBI Bookshelf) ISBN 0-9631172-1-1. 
  6. ^ Bisno AL, Brito MO, Collins CM (2003). "Molecular basis of group A streptococcal virulence". Lancet Infect Dis 3 (4): 191-200. PubMed. 
  7. ^ Starr C, Engleberg N (2006). "Role of hyaluronidase in subcutaneous spread and growth of group A streptococcus". Infect Immun 74 (1): 40-8. PMID 16368955. 
  8. ^ Buchanan J, Simpson A, Aziz R, Liu G, Kristian S, Kotb M, Feramisco J, Nizet V (2006). "DNase expression allows the pathogen group A Streptococcus to escape killing in neutrophil extracellular traps". Curr Biol 16 (4): 396-400. PMID 16488874. 
  9. ^ Wexler D, Chenoweth D, Cleary P (1985). "Mechanism of action of the group A streptococcal C5a inactivator". Proc Natl Acad Sci U S A 82 (23): 8144-8. PMID 3906656. 
  10. ^ a b Ji Y, McLandsborough L, Kondagunta A, Cleary P (1996). "C5a peptidase alters clearance and trafficking of group A streptococci by infected mice". Infect Immun 64 (2): 503-10. PMID 8550199. 
  11. ^ Hidalgo-Grass C, Dan-Goor M, Maly A, Eran Y, Kwinn L, Nizet V, Ravins M, Jaffe J, Peyser A, Moses A, Hanski E (2004). "Effect of a bacterial pheromone peptide on host chemokine degradation in group A streptococcal necrotising soft-tissue infections". Lancet 363 (9410): 696-703. PMID 15001327. 
  12. ^ a b Hidalgo-Grass C, Mishalian I, Dan-Goor M, Belotserkovsky I, Eran Y, Nizet V, Peled A, Hanski E (2006). "A streptococcal protease that degrades CXC chemokines and impairs bacterial clearance from infected tissues". EMBO J 25 (19): 4628-37. PMID 16977314. 

[edit] Other reading

  • Gladwin, Mark and Bill Trattler. Clinical Microbiology Made Ridiculously Simple, 3rd edition, 2004.
  • Brooks, Geo F., Janet S. Butel, and Stephen A. Morse. Jawetz, Melnick, and Adelberg's Medical Microbiology, 22nd edition, 2001.
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