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A. Streptococcus And Enterococcus
  1. Catalase negative, gram positive, nonsporing cocci, 1µm in diameter, some species may be capsulated
  2. Cell division occurs in one plane and cells do not separate. Therefore, are arranged in pairs or chains
  3. Aerobes and facultative anaerobes (some are obligate anaerobes)
  4. Grow poorly on basal media, growth enhanced on enriched media (addition of glucose, blood or serum)
  5. Form part of normal flora of man and animals
  6. Most important species is S. pyogenes (group A beta hemolytic streptococci)
B. Classification
  1. Based on type of hemolysis seen on blood agar divided into three categories-Brown’s classification
  2. ß-hemolytic streptococci: produce clear zone of hemolysis on blood agar. Due to production of two types of soluble hemolysins (streptolysin O & S). Most of the pathogenic species belong to this group. On the basis of group specific carbohydrate antigen in the cell wall, ß-hemolytic streptococci are divided into 21 serological groups also called as lancefield groups (A-H, K-V). S. pyogenes belongs to group A. S. pyogenes is further divided into 120 Griffith types based on surface M protein.
  3. a-hemolytic streptococci: produce narrow zone of partial hemolysis and green coloration. Also known as viridans streptococci.
  4. ?-hemolytic streptococci: Do not produce any obvious change around colonies on blood agar and are therefore nonhemolytic.
  5. Classification system not satisfactory:
    1. Certain members of group B, C, D, H are non-hemolytic
    2. Group D enterococci are non-hemolytic on sheep blood agar but ß-hemolytic on human blood agar

Streptococcus Pyogenes

A. Morphology
  1. Catalase negative, gram positive, nonsporing cocci, 1 micron in diameter; arranged in chains; some strains produce capsule composed of hyaluronic acid.
B. Cultural characteristics
  1. Aerobes and facultative anaerobes, optimum temperature 37°C. It is exacting in nutritive requirements, growth occurring only in media containing fermentable carbohydrates, blood or serum.
  2. After overnight incubation, colonies on blood agar are 05-1.0 mm in diameter, semitransparent with wide zone of ß-hemolysis.
  3. Virulent strains produce matt colonies while avirulent strains produce glossy colonies. Capsulated strains produce mucoid colonies. 
C.  Selective media:
  1. Pike’s medium, also used as transport medium (crystal violet 0.0002% in blood agar inhibits the growth of staphylococci),
  2. PNF medium (horse blood agar containing Polymyxin B, neomycin sulphate, fusidic acid). 
D. Susceptibility to physical and chemical agents
  1. Killed by heating at 54°C for 30 minutes. Killed by usual strengths of disinfectants. It is more resistant to crystal violet than other bacteria (therefore incorporated in selective media)
  2. Sensitive to benzylpenicillin and wide range of other antimicrobial agents. Naturally resistant to sulphonamides and tetracyclines. 
E.  Biochemical reactions
  1. Catalase negative, not soluble in 10% bile, hydrolyze L-pyrrolidonyl-ß-naphthylamide (PYRase test positive), ferments several sugars with production of acid but no gas. 
F.  Antigenic structure
  1. Capsule: composed of hyaluronic acid, non-antigenic as it is indistinguishable from the hyaluronic acid of connective tissue
  2. Cell wall:
    1. it is composed of an outer layer of protein, a middle layer of group specific carbohydrate and an inner layer of peptidoglycan
    2. Peptidoglycan: responsible for cell wall rigidity
    3. Group specific carbohydrate: is an integral part of the cell wall therefore has to be extracted for grouping by precipitation test with group antisera. There are various methods of antigen extraction e.g. Lancefield’s acid extraction method (using hydrochloric acid), Fuller’s method (using formamide), Maxted’s method (using enzyme produced by Streptomyces albus) and Rantz and Randall’s method (using autoclaving). On the basis of group specific carbohydrate antigen in the cell wall, ß-hemolytic streptococci are divided into 21 serological groups also called as lancefield groups (A-H, K-V).
    4. Proteins: S. pyogenes has three protein antigens (M,T and R) in its cell wall. M protein is most important. It is heat and acid stable but susceptible to tryptic digestion. About 120 M types have been recognized.
    5. Various structural components of S. pyogenes exhibit antigenic cross reaction with different tissues of the human body
Structural component of S. pyogenes Human tissue with which it cross-reacts
Hyaluronic acid Synovial fluid
Cell wall protein Myocardium
Group A carbohydrate Cardiac valves
Cytoplasmic membrane antigens Vascular intima
Peptidoglycan Skin antigens
G. Virulence factors of S. pyogenes
Virulence factor Activity
Lipoteichoic acid Mediates adherence to epithelial cells
M protein Mediates adherence to epithelial cells, inhibits phagocytosis
Hyaluronic acid capsule Inhibits phagocytosis
Erythrogenic toxin (Streptococcal pyrogenic exotoxin) There are three distinct toxins A, B, C (SPE A, B and C). Type A is encoded by bacteriophage gene speA, type B toxin by chromosomal gene speB and toxin C by bacteriophage gene speC. These toxins are superantigens , T cell mitogens that induce lymphocytes to synthesize and release cytokines which are responsible for fever shock and tissue damage
Streptolysin O In addition to group A streptococci, it is also produced by some group C and G streptococci. It is a heat labile protein. It acts by binding to cholesterol in the cell membrane and producing holes in it. It is inactivated by oxygen. It is strongly antigenic and demonstration of antibodies (Antistreptolysin O) against it indicates recent group A streptococcal infection.
Streptolysin S This hemolysin is not inactivated by oxygen and is responsible for hemolysis on the surface of an aerobic blood agar plate. Due to its small size it is not antigenic. It also has leucocidal action.
Streptokinase Produced by group A, C and G streptococci and is antigenic. It has fibrinolytic action and is thought to be responsible for rapid spread of infection by preventing the formation of fibrin barrier. This property is also used therapeutically in treatment of coronary thrombosis.
Deoxyribonuclease Four different types have been identified (A,B,C and D). They hydrolyze nucleic acids and nucleoproteins. They are antigenic and DNAase B is the most common form produced by S. pyogenes. Anti-DNAase B is useful for the retrospective diagnosis of skin infections, where antistreptolysin titre may be low.
Hyaluronidase Produced by strains of group A, B, C and G streptococci. It is antigenic. It is thought to play an important role in spread of infection through the tissues.
Serum opacity factor It is a lipoproteinase enzyme. It reacts with and produces opacity in mammalian sera. It is produced by group A streptococci of certain M types. It is loosely bound to the cell and is antigenic.
H. Pathogenicity
  1. Source of infection: Carriers and patients with acute infection
  2. Infection is acquired by respiratory droplets or by direct or indirect contact
  3. Suppurative diseases
    1. Tonsillitis and pharyngitis: Most common cause of bacterial sore throat. From the throat infection may spread resulting in complications such as peritonsillar abscess (quinsy), otitis media, sinusitis, mastoiditis. Infrequently diffuse cellulitis of the floor of mouth (Ludwig’s angina) can develop. Bronchopneumonia can develop as a result of viral infections of the respiratory tract such as influenza and measles.
    2. Scarlet fever: Caused by group A streptococcus producing erythrogenic toxin. This syndrome is   characterized by streptococcal sore throat with generalized punctate erythema or rash.
    3. Impetigo: most common cause. Apart from group A streptococcus it may also be caused by   group C & G streptococci.
    4. Erysipelas: This is acute spreading intensely erythematous skin lesion with sharply demarcated edges. It may be preceded by streptococcal sore throat or infected wound.
    5. Streptococcal toxic shock syndrome: It is characterized by bacteremia, toxemia, fever, shock, sever pain at the site of infection. There may be acute respiratory distress, renal failure. Mortality is between 20-30%. S. pyogenes types 1,3,12 and 28 are commonly involved.
    6. Necrotizing fascitis: most common cause
    7. Other infections: S. pyogenes is a most important cause of puerperal sepsis. It may also cause wound or burn infections.

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