Written by MicroDok

Streptococcus pneumoniae is a Gram-positive, capsular, non-motile and catalase-negative cocci that are usually lancet (bullet) shaped. S. pneumoniae also occurs in pairs (diplococci) or in chains (streptococci); and they are haemolytic in nature and inhibited by optochin. A member of the normal flora of the human upper respiratory tract system, S. pneumoniae (commonly referred to as pneumococcus), is notorious in causing a range of infections in human beings. It is the most common cause of community acquired pneumonia in humans. Other infections in which S. pneumoniae is implicated are osteomyelitis, abscesses, otitis media, meningitis, bacteraemia, peritonitis, endocarditis and cellulitis.

Virulent strains of S. pneumoniae have surface capsules composed mainly of high-molecular monosaccharides and oligosaccharides that interfere with phagocytosis. The upper respiratory tract of humans is the main reservoir of pneumococci, and they can be spread to susceptible host via respiratory droplets through this route. Pneumococci colonize the upper respiratory tract without causing any disease, but infection ensues following weakened immune system and poor state of health. Pneumonia caused by S. pneumoniae is most prevalent amongst certain individuals with some predisposing factors such as prior viral infection of the respiratory tract, history of alcoholism, drug intoxication, injury to the respiratory tract, heart failure, diabetes and old age amongst others.


Pneumonia (reduced function of the lungs) is a lung disease that is caused by certain pathogenic bacteria species including S. pneumoniae (pneumococcus). Other bacteria that cause human pneumonia are Staphylococci, Haemophilus, Pseudomonas, Mycoplasma and Chlamydia. Non-bacterial causes of pneumonia are some species of viruses, fungi and protozoa. The virulence of infection caused by S. pneumoniae is usually determined by some virulent factors such as pili, capsules, cell wall surface proteins (teichoic acid and peptidoglycan), haemolysins, hydrogen peroxides and autolysins. The onset of pneumonia in humans starts following the aspiration of respiratory secretions that contains virulent strains of S. pneumoniae.

Upon invasion and possible colonization of the upper respiratory tract where they normally inhabit, S. pneumoniae reaches the lower respiratory tract where they attack the alveolar cells and produce a variety of lung diseases. S. pneumoniae multiplies rapidly in the alveolar spaces, and produce inflammatory cells which fills the alveoli and/or lungs. Systemic infections due to pneumococcus occur when S. pneumoniae reaches the body’s blood circulation via the lymphatic vessels of the lungs. S. pneumoniae can reach other sites of the body such as the middle ear, heart and meninges via the respiratory tract and produce further clinical complications. Fever, chills, chest pain, difficulty in breathing and expectoration of sputum usually accompanied with blood (i.e. rusty coloured blood) are some of the clinical signs and symptoms associated with pneumonia caused by S. pneumoniae.

Bronchial pneumonia and lobar pneumonia are the two types of pneumonia caused by pneumococcus in humans. Bronchial pneumonia is common in old people, young children and infants while lobar pneumonia is commonly experienced by young adults. Uncapsulated strains of pneumococcus are generally avirulent, and are not capable of producing pneumonia in humans. Only the capsulated S. pneumoniae strains actually produce the clinical episodes of the disease, and this is because capsulated pneumococci have polysaccharide capsules that protect them from phagocytosis.     


The laboratory diagnosis of pneumonia caused by S. pneumoniae is mainly based on the identification and isolation of the pathogen from patient’s specimens via microscopy and culture techniques. Sputum, CSF, exudates and blood specimens are usually the main samples collected when pneumococcal pneumonia is suspected. Gram stained smear of sputum samples reveals Gram-positive lancet (bullet) shaped diplococci. S. pneumoniae is a fastidious bacterium, and they grow best in culture media supplemented with horse, rabbit or human blood and incubated in 5% carbon dioxide.

On blood agar, S. pneumoniae produce mucoid or translucent colonies that are alpha haemolytic; and pneumococcus is optochin sensitive and soluble to bile salts. Biochemically, pneumococcus ferment glucose to produce lactic acid. Serologically, S. pneumoniae is positive to the quellung reaction test also known as the swelling reaction. In quellung reaction test which is used to determine the polysaccharide capsule of S. pneumoniae, the capsules of pneumococcus swells markedly when the pathogen is mixed with certain specific antiserum and then examined microscopically at 1000X for capsular swelling. Quellung reaction test is usually used for the rapid detection of pneumococcus from cultures and sputum samples in the microbiology laboratory.


Pneumonia caused by pneumococcus is stopped abruptly when the right antimicrobial agents are administered early. Penicillins V & G, vancomycin, cephalosporins, macrolides and quinolones are some of the drug classes used to treat and manage pneumococcal pneumonia. Some strains of pneumococcus may be resistant to some first-line antibiotics such as penicillins, thus antibiotic therapy should be guided by the results of susceptibility studies.


The prevention of pneumococcal pneumonia is largely based on immunization using pneumococcal vaccine and the effective treatment of affected individuals.


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