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.

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.