Streptococcus Pneumonia

INTRODUCTION:
  • Also known as pneumococci
  • Gram positive
  • Lanceolate shaped diplococci
  • Possess a specific polysaccharide capsule
  • Produce milk borne diseases
  • commonest Post splenectomy infection
  • Normal inhabitants of human upper respiratory tract
  • Causes pneumonia, otitis media in children, sinusitis, meningitis etc
MORPHOLOGY:
  • Typically small (1µm)
  • Elongated cocci with one end broad and other end pointed presenting a lanceolate appearance
  • They are capsulated enclosing each pair
  • Non motile and Non sporing
  • Readily stained with aniline dyes
  • capsules may be demonstrated as clear halo in India ink preparations
CULTURAL CHARACTERISTICS:
  • Grow only in enriched media
  • They are aerobes and facultative anaerobes
  • Optimum temperature 37 oC (25-42) and ph of 7.8 (6.5-8.3)
  • On blood agar, after incubation for 18 hrs, the colonies are dome shaped and glistening with an area of green discolouration(alpha hemolysis) around them resembling streptococcus viridans. On further incubation they become flat with raised edges and central umbonation (Draughtsman or carom coin appearance)
  • Under anaerobic conditions, colonies on blood agar are surrounded by a zone of beta hemolysin O.
  • In liquid media such as glucose broth, growth occurs as uniform turbidity the cocci readily undergoes autolysis which is enhanced by bile salts etc
BIOCHEMICAL REACTIONS:
  • They ferment several sugars, forming only acid .
  • Fermentation is tested in Hiss’s serum sugars.
  • S. pneumonia are catalase and oxidase negative.
  • Fermentation of inulin is a useful test for differentiating them from streptococci, as the latter do not ferment it.
  • Bile solubility test:
  • S. pneumonia are bile soluble.
  • If a few drops of 10% Na deoxycholate soln are added to 1ml of an overnight broth culture, the culture clears due to lysis of cocci .
  • Bile solubility is a constant property of S. pneumoniae and hence is of diagnostic importance.
  • Test should be done at neutral ph using deoxycholate and live young cell in saline suspension
  • Principle: bile solubility is due to presence of an autolytic amidase that cleaves the bond between alanine and muramic acid in the peptidoglycan.
  • The amidase activated by surface active agents like bile or bile salts.
RESISTANCE:
  • Readily destroyed by heat (thermal death point 52 oC for 15 min) and antibiotics.
  • In culture they die on prolonged incubation due to accumulation of toxic peroxides.
  • Strains may be maintained on semi solid blood agar or by lyophilisation.
  • Sensitive to most antibiotics beta lactams being the drug of choice.
  • Optochin sensitivity:
  • Sensitvity of S.pnuemoniae to optochin (ethyl hydrocuprein) is useful in differentiating them from streptococci.
  • When a disc impregnated with optochin is applied on a plate of blood agar inoculated with S.pneumonia , a wide zone of inhibition appears on incubation .

Specimen(CSF/blood)
Plate on blood agar
Identify alfa-hemolytic colonies
Gram stain Catalase test Optochin test
Identify Gr.+ve diplococci or Gr+ve cocci in short chain Catalase -Ve
Optochin susceptible Optochin Resistant
Streptococcus sp. from gram stain & catalase result
S. pneumonia(Gr.+ve, Catalase -ve),Bile solubility test
Determine serotype Soluble Insoluble
(S.pneumonia),(α-hemolytic other than S.pneumonia)
Determine serotype
ANTIGENIC PROPERTIES:
  • CAPSULE :
  • The most important antigen of S.pneumonia is the type specific capsular polysaccharide( antibody against it is IgG subclass 2)
  • As this polysaccharide diffuses into the culture medium or infective exudates and tissues , it is also called “specific soluble substance” (SSS)
  • S.pneumonia are classified based on antigenic nature of capsular polysaccharide .
  • More than 90 serotypes are recognized named 1,2,3 .. etc
  • SEROTYPING based on capsular antigens may be carried out by:
  1. Agglutination of cocci with type specific serum
  2. Precipitation of the SSS with specific serum
  3. By capsule swelling or “QUELLUNG REACTION” described by Neufeld. Here a suspension of S.pneumonia is mixed on a slide with a drop of type specific antiserum and a loopful of methylene blue soln. In the presence of homologous antiserum, the capsule becomes apparently swollen, sharply delineated and refractile. The quellung reaction test can be done directly with sputum from pneumonia cases.
  4. PCR based tests :These test have shown higher sensitivity in detection of infection esp in meningitis as it can detect the presence of a small number of the specific DNA sequences of bacteria which cannot be cultured by conventional methods due to administration of prior antibiotics or because of a smaller bacterial load in body fluids.
OTHER ANTIGENS
  • A nucleoprotein deep inside the cell and a somatic ‘C’ carbohydrate antigen both of which are species specific ‘C’ ANTIGEN OF BACTERIAL CELL WALL AND CRP
TOXINS AND VIRULENCE FACTORS:
  • The virulence of S.pneumonia depends on :
  1. The capsular polysaccharide, because of its acidic and hydrophilic properties, the cocci protected from phagocytosis. But they are susceptible to ‘surface phagocytosis’, being engulfed against a firm surface such as clot or epithelium.
  2. The enhanced virulence of type3 S.pneumonia is due to abundance of capsular material
  3. Pneumolysin: a membrane damaging toxin produced by the cocci has cytotoxic and complement activating properties. Pneumolysin –ve mutants show reduced virulence.
  4. Autolysin: these helps in releasing bacterial components and toxins like pnemolysin and thus contribute to virulence
  5. Oxygen labile hemolysin and leucocidin – weak , produce no virulence
DISEASES:
NON-INVASIVE DISEASES:
  1. SINUSITIS
  2. OTITIS MEDIA (middle ear)
  3. PNEUMONIA (lungs)(lobar & community acquired commonly)
INVASIVE DISEASES( bacteremia)
  1. MENINGITIS (CNS)
  2. ENDOCARDITIS (CVS)
  3. PERITONITIS (body cavity)
  4. SEPTIC ARTHRITIS
  5. UVEITIS
  6. SUBDURAL EMPYEMA
  7. OTHERS (appendicitis, salpingitis, soft tissue infections)
Exam Question
  • Alpha-hemolytic colonies inhibited by optochin on blood agar and lysed by bile is the characteristic feature on culture of Streptococcus pneumoniae organism
  • Antibody against organisms having polysaccharide antigens i.e Streptococcus pneumoniae is IgG subclass 2
  • Milk borne diseases are can be caussed by Salmonellosis ,E. coli, Streptococcus
  • Uveitis is caused by streptococcus pneumoniae
  • Streptococcus Pneumoniae is the commonest Post splenectomy infection
  • Common organisms causing sinusitis streptococcus pneumoniae
  • Streptococcus pneumoniae is the most common causative organism for lobar pneumonia
  • Subdural empyema is most commonly caused by Streptococcus pneumoniae
  • SABE is most commonly due to Streptococcus pneumonia
  • Community acquired pneumonia is most commonly caused by Streptococcus pneumoniae
  • Lobar pneumonia is caused by Streptococcus pneumoniae
  • Past history of ear infection with susceptibility of meningitis is associated with streptococcus pneumoniae
  • Streptococcus pneumonia produces Alpha type of hemolysis
  • Most common infection caused by streptococcus pneumonia Otits media
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