Bacteriology- Streptococcus | Quick Revision Notes for NEET-PG

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Peptostreptococcus-anaerobius Magnus is present in the oral cavity like female genitalia tract and GIT and causes lung abscess, brain abscess, etc

Group A streptococci/GAS is bacitracin sensitive, all others beta-hemolytic streptococci are resistant.

Morphology- Gram-positive cocci arranged in chains/pair(division in 1 plane)

It is Catalse -ve(Catalase test is the most important test to differentiate between Staphylococcus and Streptococcus)


Fastidious in nature



  • On blood agar, it produces pinpoint translucent non-pigmented colonies surrounded by beta hemolysis.
  • Selective media-crystal violet blood agar
  • Transport media is PIKE’S media

Virulence Factors

  1. Lipoteichoic acid- mediates adherence of bac. to host epithelial cell
  2. Capsule containing hyaluronic acid (non-phagocytic)
  3. Protein M(anti-phagocytic)
  4. Streptolysin hemolysin -It is of 2 types

Streptolysin hemolysin

  • O type

Streptolysin hemolysin

-S type


Heat stable

oxygen labile( seen only in deep colonies

 as it is inactivated in the oxidized state)

Oxygen stable

strongly antigenic


ASO (Antistreptolysin O antibody titer (Std. Marker)

is diagnostic for Streptococcal infection.

Not useful for serological Dx


It has leucocidal activity.

  1. Streptococcal pyrogenic Exotoxin/SPE/erythrogenic cycle/scarlet fever toxin-Induces 3 types(a,b,c) of fever. SPE-A, C are superantigens coded by bacteriophage and act as T cell mitogens releasing abundant amt. of cytokines in return causing fever, shock and tissue debris
  2. Collagenase
  3. Streptokinase(Fibrinolytic)-helps in the spread of infection to skin and subcutaneous layer by activating plasminogen to plasmin and helps in breaking down fibrin clot. It is antigenic in nature and produced by A, C and G streptococci.
  4. Hyaluronidase

FIGURE- Cell wall of S. pyogenes


Suppurative-Pharyngitis (sore throat)-MC bacterial cause of a sore throat. Complication of a sore throat include Peritonsillar abscess and  Ludwig’s angina(infection of submandibular space)

  • Impetigo(superficial infection of skin most commonly involving face and legs)
  • Cellulitis (infection involving skin and subcutaneous tissue)
  • Necrotizing fasciitis
  • Scarlet fever (Pharyngitis with rash-feels like sandpaper, rashes in skin folds k/a Pastia’s line, strawberry tongue, susceptibility testing by Dick test (erythematous lesions upon intradermal injection of streptokinase/fibrinolysin).


  • Acute rheumatic fever -It is caused by all serotypes and develops 1-5 weeks after pharyngitis.
    • It is a type 2 hypersensitivity rxn with cross mimicry involving the formation of antibodies against GAS to cross-react with human tissue antigen(present in heart valves, brain, joint) or direct destruction of the cardiac cell by SPE/ SL-O.
    • Some of the Clinical manifestations include pancarditis, polyarthritis, Aschoff nodules, etc.
  • Acute Post streptococcal glomerulonephritis-It develops 2-4 weeks after impetigo/sore throat. As the kidney receives 25% of cardiac output, it also filters IgG Abs against GAS Antigen which then makes immune complexes with the latter. These immune complexes deposit in glomeruli leading to glomerulonephritis accompanied by edema, oliguria and GAS casts in urine.

Group B streptococci/ S. Agalactiae

Present as commensals in the female vagina. Infection can be transmitted from mother to baby during vaginal/normal delivery.

It is identified by 2 tests –

  1. Hipurrate test
  2. AMP test(Christie-Atkins-Munch-Petersen test)- synergistic arrow shaped Beta hemolysis produced by Streptococcus aureus and cAMP factor(produced by GBS)when GBS is smeared on blood agar plate perpendicular to S. Aureus.

Diseases- MC cause of neonatal septicemia and neonatal meningitis in the west.

Drug of choice is penicillin G and if the patient is allergic to penicillin then give him/her erythromycin or azithromycin.

Viridans streptococci

They are commensals of mouth and upper respiratory tract.

They are MC cause of subacute bacterial endocarditis. They also cause dental carries with biofilm production(mainly by S. mutan), brain abscess, etc.

Extra- diagnosis of Rheumatic fever-


Fig1- Cell wall of S. pyogenes Fig2 - CAMP test

 Fig2 - CAMP test

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