Streptococcus pyogenes
streptococci
cocci means - round or spherical shaped bacteria
streptos means - twisted or coiled in appearance
the name pyogenes is given because it is found in suppurative lesions (that is pus-forming diseases)
Therefore the name streptococci pyogenes.
FEATURES
- either aerobic, facultative anaerobes or obligate anaerobes
- aerobic and facultative anaerobes alone have hemolytic properties
- based on the hemolytic properties we have alpha, beta and gamma bacteria
-the alpha bacteria show partial hemolysis therefore a green discoloration is seen
-the beta hemolytic bacteria show complete hemolysis and colorless zone is seen in the blood agar culture medium
- whereas the gamma type of bacteria does not show any hemolytic properties and are non-hemolytic
The beta hemolytic streptococci are again divided into 20 groups by Lancefield and these 20 groups are classified based on these C carbohydrate antigen present on the cell wall of the bacteria.
Lancefield group A which we commonly call as Streptococcus pyogenes was again subdivided into 1 to 80 subtypes - which were given by Griffith based on the surface proteins MTR
- group A of streptococci contributes to majority of the human infections among the Streptococci.
MORPHOLOGY
spherical or ovoid - as the name cocci suggests
gram positive
non-motile
non-sporing
diameter - from 0.5 to 1 micrometer
capsule may be present in some strains either made of hyaluronic acid or polysaccharide
chain formation is the classical feature of Streptococcus pyogenes - it happens because after cell division daughter cells are attached to one and one to another and does not separate completely.
REQUIREMENTS
ideal temperature is 37 degrees Celsius the range varies from 22 to 42 degree celsius
nutritional requirement includes either blood, serum or fermentable carbohydrates
blood agar - ideal because the hemolytic property of the bacteria can be demonstrated in blood agar
- if you leave the bacterial strains overnight in blood agar- colorless zone of complete hemolysis appears that is associated with the beta hemolytic strains
- the colorless zone ranges from 2 to 4 millimeter wide
(whereas the colonies range from 0.5 to 1 millimeter wide -they are pinpoint in appearance) and around these colonies RBCs are destroyed.
This happens because of this streptolysin O and S
O stands for oxygen labile strain and S stands for oxygen stable streptolysin
Other features of the colonies -
the colony is semi-transparent
it is low convex and if large capsules are present the mucoid nature of the colonies can be appreciated
The selective media
Crystal Violet blood agar
PNF medium
THE BIOCHEMICAL REACTIONS:
-which are useful in differentiating this streptococcus pyogenes from the other bacteria
- S.pyogenes is catalase negative whereas the Catalase test is positive in staphylococcus
-Streptococcus pyogenes is not soluble in 10% bile whereas the streptococcus pneumoniae is soluble
- Pyrase test positive - it produces deep red color and this differentiates it from the other streptococci which produce a diffuse orange color
ANTIGENIC STRUCTURE of Streptococcus pyogenes
capsule (orange layer in picture)- the outer component (the group A and C of the Lancefield classification have hyaluronic acid as the capsule) it inhibits phagocytosis - therefore it is an important virulence factor.
Whereas the second, third and the fourth layer that constitutes the cell wall.
- the pink layer denoted here is made of protein and lipoteichoic acid - it contains pili for attachment to the host
-the middle layer - made up of group specific carbohydrate with which Lancefield classified the bacteria into 20 groups
- the third layer - the inner layer is made of peptidoglycan - it is responsible for the cell wall rigidity and is also responsible for the pyrogenicity - that is the ability to induce fever and thrombolytic capability of this bacteria
- the outer part of the cell wall has some type specific antigens - M protein, T and R protein.
- M protein is heat and acid stable it is susceptible to trypsin and it is one of the major virulence factor.
- the antibody to M protein is protective but it plays a role in rheumatic fever pathogenesis.
(Acute rheumatic fever is a non-suppurative complication of streptococcus -which occurs up to four weeks after the initial streptococci infection.)
- M protein is known to inhibit phagocytosis
- T and R proteins have no role in virulence they are acid labile and trypsin resistant, in contrast to the M protein.
TOXINS
Hemolysin (Streptolysin) and Pyrogenic exotoxin
Hemolysin O and S both are responsible for the beta hemolysis
O is oxygen labile
S is oxygen stable
Therefore O is present only in the deep colonies and it is antigenic in nature
anti-streptolysin O - ASO titres useful in laboratory diagnosis
It is heat labile.
Streptolysin S is not antigenic in nature.
Hemolysin O is also cytotoxic in nature.
It affects WBC platelets and cardiac tissue whereas S is associated with hemolysis and leucocidal property.
Pyrogenic exotoxin
the name pyrogenic exotoxin means that it induces fever and it is also known as erythrogenic toxin (this name was given because it was responsible for the rash of scarlet fever and this toxin is currently named as streptococcal pyrogenic exotoxin SPA)
ENZYMES
the enzymes released by streptococcus pyogenes include
- streptokinase
- DNASE
- NADASE
- hyaluronidase and others
HYALURONIDASE
-it is antigenic in nature and it facilitates spread of infection.
-it destroys the hyaluronic acid
STRPTOKINASE
- streptokinase is also antigenic in nature and anti-streptokinase appear in serum which is helpful in diagnosis
- it is also known as fibrinolysin because it catalyzes the transfer of plasminogen to plasmin that is it helps in the lysis of the human fibrin clot Therefore fibrinolysin denotes the function of it and it has pharmaceutical purpose also.
It facilitates spread of infection
DNASE - deoxyribonuclease also known as streptodornase
- it has four antigenically distinct types among which B is most antigenic
- it liquefies DNA
NADASE
it liberates nicotinamide by acting on NAD
it is leucotoxic in nature
other enzymes include
- neuraminidase
- esterase
- amylase
- lipase and
- Beta glucuronidase
PATHOGENESIS
Pathogenesis of Streptococcus pyogenes infection is a pyogenic (that is pus producing infection) and it spreads locally as we saw because of streptokinase, hyaluronidases. Some of the common diseases include,
-The pyogenic infections of respiratory tract
-acute tonsillitis
-pharyngitis
-scarlet fever (which is rare in recent days)
- infection of wound like Burns
- eczema
- impetigo and
- necrotizing fasciitis (one of the common spreading infections)
- Streptococcal toxic shock syndrome is associated with SPA strain and entire organ system collapse is seen. It is actually a life-threatening condition. Bacteremia is one of the feature.
Non-suppurative complications
- acute rheumatic fever and
- acute glomerulonephritis
Acute rheumatic fever is preceded usually by sore throat whereas glomerular nephritis is proceeded by skin infection. It appears one to four weeks after the initial infection.
The main property of these kind of non-suppurative complications are the cross reacting antibody -the antibodies produced in response to streptococcus cross react with the antigens present in kidney and heart.
LABORATORY DIAGNOSIS:
The laboratory diagnosis of streptococcal is mainly by the specimen collected and cultured.
specimens are collected either by pus, swab, blood or CSF depending on the site of infection using a sterile container and if it is need to be transported Pikes transport medium is used.
Gram staining - gram positive cocci which appears as chains and in culture with sheep blood agar shows hemolysis.
- it is anaerobic
- 37 degree Celsius with 5 to 10 percent carbon dioxide is the ideal condition to grow
- Liquid media culture shows granular turbidity with powdery deposits. Which is point to remember and
TREATMENT
Penicillin G is the drug of choice ,amoxicillin is also used.
In case of penicillin allergy, erythromycin is used.
Cephalexin is also used.
PROPHYLALXIS
In order to help in the prevention of rheumatic fever in children with early signs of rheumatic fever, long period administration of penicillin is advised.