Streptococcus

INTRODUCTION:
  • Gram positive cocci
  • Arrangement : chains or pairs
  • Part of normal flora
  • Causes pathogenic infections
  • Phylum Firmicutes and the order Lactobacillales (lactic acid bacteria).
  • Most are oxidase-negative and catalase-negative, and many are facultative anaerobes.
  • Cell division along a single axis in these bacteria (Contrast this with staphylococci, which divide along multiple axes and generate grape-like clusters of cells.)
CLASSIFICATION:
  1. Brown's classification
  2. Lancefield grouping
  3. Griffith typing
Brown's classification:
  • α- hemolitic( green)
  1. Streptococcus pneumoniae( optochin sensitive, bile soluble, capsule=>quellung+)
  2. Streptococcus viridans(mutans,sanguis optochin resistant,no capsule, bile insoluble)
  • 2. β- hemolitic(clear)
  1. Streptococcus pyogens( group A, bacitracin sensitive)
  2. Streptococcus agalactiae( group B, bacitracin resistant)
  • 3. γ-hemolitic: Entrococcus
  1. E. faecalis
  2. E. faecium
Lancefield grouping:
  • Hemolysis on agar plates containing sheep blood:
  • Lancefield groups(A,B.....T) Major cell-wall carbohydrate antigens
α and non-hemolytic, γ
  1. Group D( heat resistant grows at temperature>60oC)
  2. S. pneumonia
  3. S. Viridans
  4. m
2. β :
  1. Group A(most commonly causes cellulitis)
  2. Group B S. Agalactiae(lives in female genital tract)(CAMP test is positive)
  3. Group C,G
SPECIES OF STREPTOCOCCUS & ASSOCIATED DISEASE:
  • Streptococcus causes destruction of skin grafts
Species Host Disease
S. pyogenes human pharyngitis, Scarlet fever,Erysipelas,Impetigo, cellulitis,Pseudomembranous conjunctivitis
S. agalactiae human, cattle neonatal meningitis and sepsis,Pseudomembranous conjunctivitis
S. dysgalactiae human, animals endocarditis, bacteremia, pneumonia, meningitis, respiratory infections
S. bovis human, animals biliary or urinary tract infections, endocarditis
S. anginosus human, animals subcutaneous/organ abscesses, meningitis, respiratory infections
S. sanguinis human endocarditis, dental caries
S. suis swine meningitis
S. mitis human endocarditis
S. mutans human dental caries
S. pneumoniae human pneumonia
CAPSULE:
  • Some strains of streptococcus pyogenes form a capsule composed of hyaluronic acid. Such strains grow as mucoid colonies on blood agar.
  • The capsule is identical to the hyaluronic acid of the connective tissue of the host and is not immunogenic.
  • Like other bacterial capsule it has an antiphagocytic effect.
VIRULENCE FACTOR:
M. protein
  • It is the major virulence factor
  • It is the main factor responsible for resisting phagocytosis
  • The ability of streptococcus pyogenes to resist phagocytosis by polymorphonuclear leucocytes is to a large degree due to the M. Protein.
  • Resistance to infection by streptococcus is acquired as a result of antibodies to the M. Protein.
Exotoxins secreted by the bacteria
Pyrogenic exotoxin
  • Most strains of streptococcus pyogenes produce one or more toxins
  • that are called pyrogenic exotoxins because of their ability to induce fever.
  • Three types of streptococcal pyrogenic exotoxins have been identified, i.e. A, B, and C.
  1. The genes of SPE "A" and SPE 'C' are transmitted between the strains of streptococcus by bacteriophage° and stable production depends on lysogenic conversion".
  2. The genes for SPE B are coded by chromosomes(not plasmids)
  3. SPE A and SPE C' are also called erythrogenic toxins as they are responsible for rash observed in patients with scarlatina.
  4. They have superantigen activity which causes massive release of inflammatory gtokines from T lymphocytes causing fever, shock and tissue damage.
  5. Pyrogenic exotoxin A :Gene transmitted by phase
  6. Pyrogenic extoxin B :Gene transmitted by chromosome
  7. Pyrogenic exotoxin C :Gene transmitted by fi phase
Streptolysins:
Streptococcus pyogenes produce two distinct hemolysins termed
  • Streptolysin 0 (oxygen labile)
  1. Inactive in the presence of oxygen.
  2. It can cause hemolysis in "reduced state only".
  3. Immunogenic.
  4. Pathogenic of post streptococcal rheumatic fever.
  5. ASO titre is standard serological procedure for retrospective diagnosis of infection with 
  • streptococcus pyogenes.
  1. Streptolysin 'S' (oxygen stable)
  2. 13 hemolysis around colonies on blood agar plates (because streptolysin '0' cannot hemolyze).
  3. Non- immunogenic.
Streptokinase:
  • It is also known as fibrolysin.
  • It is expressed on all strains of streptococcus pyogenes including the A,C,K..
  • It is expressed close to plasminogen binding site on the bacteria, when host plasminogen is bound to the bacterial surface, it is activated to plasmin by streptokinase
. Deoxyribonuclease (streptodornase, DNAase):
  • There are at least four distinct forms of DNAases designated as:
  1. DNA ase A
  2. DNA ase B
  3. DNA ase C
  4. DNA ase D
  • DNA ase B is the most common form
  • "These enzymes hydrolyze nucleic acids".
Hyaluronidase
  • This enzyme breaks down the hyaluronic acid of the tissue.
  • This might favour the spread of infection along the intercellular spaces.
Serum opacity factor
  • Some M types of streptococcus pyogenes produce a lipoproteinase which results in opacity when applied to agar gel.
DIFFERENTIAL DIAGNOSTIC TEST:
Catalase test is used to differentiate staphylococci from streptococci.
Pneumococcus can be differentiated from streptococcus by Bile solubility test
Exam Question
  • Streptodornase cleaves DNA
  • Streptolysin 0 is active in reduced state
  • Streptokinase is produced from serotype A,C,K
  • M protein is responsible for virulence
  • Mucoid colonies are virulent
  • M protein is the major surface protein of group A streptococci
  • Endotoxin causes rash of scarlet fever
  • Catalase test is used to differentiate staphylococci from streptococci.
  • C-carbohydrate in Streptococcus hemolyticus is important for Lancefield classification
  • Streptolysin causes hemolysis
  • Group 'B' causes neonatal meningitis
  • Among streptococci, enterococcus (group D streptococcus) is heat resistant grows >60oC
  • Pneumococcus can be differentiated from streptococcus by Bile solubility test
  • GroupAstreptococcuscausesScarlet fever,Erysipelas,Impetigo,pharyngitis,cellulitis,Pseudomembranous conjunctivitis
  • Group B lives in female genital tract
  • Commonest cause of cellulitis is Streptococcus
  • Sreptococcus causes destruction of skin grafts
  • CAMP test is positive for Group B streptococcus
  • Streptococcus causes non­granulomatous uveitis
Don't Forget to Solve all the previous Year Question asked on Streptococcus