Disease caused by Streptococcus pneumoniae

The Streptococcus pneumoniae is gram-positive lancet shape bacteria. It may be diplococcus or streptococcus. Pneumococcus is non-motile, non-spore forming capsulated bacteria. They may be aerobic or facultative anaerobic in nature. They are a normal inhabitant of human respiratory tract Pneumococcus bacteria may be an important cause of pneumonia, otitis, sinusitis, bronchitis, meningitis and other infection like arthritis, conjunctivitis and suppurative lesions.

Streptococcus pneumoniae

Streptococcus pneumoniae

1. Morphology

  • Pneumococci are lancet shape with one end broad or rounded and other end pointed
  • If they occur in pair called as diplococci.
  • They are smaller than 1 micrometre in size.
  • The capsule enclose each pair of cells. These capsules can be made visible by using capsule staining method.
  • These cells are readily stained by gram staining procedure.

2. Cultural characters

  • Pneumococci require enriched culture for its growth.
  • They may require anaerobic or facultative anaerobic condition for growth.
  • The optimum temperature for its growth is 37° C and can grow in the temperature range of 25° C to 42° C.
  • The optimum pH -7.8 and can grow at a pH range of 6.5 to 8.3.
  • Dome shape colonies with an area of green decolourisation occur after incubation on blood agar and on further incubation colonies become flat.
  • Pneumococcus colonies on blood agar under anaerobic condition show zone of beta haemolysis.
  • In glucose broth, uniform turbidity is observed.
  • The cocci undergo autolysis by intracellular enzymes.

3. Biochemical reaction

  • Pneumococci ferment inulin and this fermentation differentiates Pneumococcus from other Streptococci.
  • It also ferments many sugars result in the production of acid and no gas.
  • Pneumococci solubilize bile constantly so it is important for diagnosis.
  • It shows catalase and oxidase test negative.

4. Resistance

  • Pneumococci are heat sensitive organism its thermal death point is 52 °C for15 minutes.
  • It is sensitive to antibiotics.
  • Prolonged incubation kills bacterial cells due to the production of toxic peroxides.
  • These strains are sensitive to many antibiotics like penicillin.

5. Toxins and other virulence factors

  • The virulence depends on capsule and toxin production called as pneumolysin.
  • The capsular polysaccharide of Pneumococci is acidic and hydrophilic in nature so these cells are protected from phagocytosis.
  • Pneumococci produce pneumolysin it is the membrane-damaging toxin and shows cytotoxicity and complement activating protein.

6. Pathogenicity

  • Intraperitoneal inoculation in mice and rabbit can cause a fatal infection.
  • Pneumococcus can localise in the human nasopharynx and can infect middle ear and respiratory tract.
  • Generally, infection is endogenous but highly virulent strains can also cause exogenous infections.
  • Pneumococci are a cause of pneumonia and bronchopneumonia.

7. Epidemiology

  • The transmission of infection takes place by inhalation of droplets contaminated with Pneumococci.
  • The carrier is the respiratory tract of patient or droplet nuclei.
  • Disease result when the immunity of a person is low and the person gets infected with Pneumococci.

8. Laboratory diagnosis

  • The sputum sample is collected from the patient.
  • The sputum sample is homogenized and gram staining procedure is carried out.
  • After homogenization, it is inoculated on blood for 37  °C for 24 hours.
  • In case of infants serum coated laryngeal swaps are used.
  • Gram-stained CSF is used for presumptive diagnosis and gram-positive diplococci are observed.

9 . Treatment

  • Antibiotic used for the treatment are penicillin and amoxicillin.
  • If the strains are resistant to other antibiotics like erythromycin and tetracycline are used.
  • In serious infections, vancomycin and cephalosporin drugs are of choice.

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