STREPTOCOCCUS PNEUMONIAE: pathogenesis, Lab diagnosis, treatment

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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 (as diplococci) or in chains (as 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 population. 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 hosts via respiratory droplets or aerosols emanating from the respiratory secretions of infected individuals. 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, and injury to the respiratory tract, as well as heart failure, diabetes and old age. People with these underlying health conditions are more prone to infection with pneumococcus than those whose health status are still intact and have strong immune system.             

PATHOGENESIS OF STREPTOCOCCUS PNEUMONIAE INFECTION

Pneumonia (a reduced function of the lungs) is a lung disease that is caused by certain pathogenic bacteria species including S. pneumoniae (pneumococcus). Other bacteria species 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 released by the pathogen during an infection. The onset of pneumonia in humans starts following the aspiration of respiratory secretions or droplets 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 of the lungs and produce a variety of lung diseases. S. pneumoniae multiplies rapidly in the alveolar spaces, and produce inflammatory cells which fills the alveoli spaces 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.     

LABORATORY DIAGNOSIS OF STREPTOCOCCUS PNEUMONIAE INFECTION

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. 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 a magnification of 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. 

IMMUNITY TO STREPTOCOCCUS PNEUMONIAE INFECTION

Immunity against pneumococcus is strain-type specific. There are about 90 different serotypes or antigenic variants of pneumococcus. Protection against pneumococcal pneumonia or any of the pathogenic strains of the pathogen is usually based on intact phagocytic function of the host’s immune system, and on antibody production against the capsular polysaccharide of S. pneumoniae. Antibody production by the host enhances phagocytosis or opsonization of the invading bacteria or pneumococcus. However, a lasting natural form of immunity after prior infection cannot be established, which is why pneumococcal pneumonia can occur throughout a person’s lifetime especially in young adults, the elderly and people with a debilitated immunity or other forms of underlying disease conditions.

TREATMENT OF STREPTOCOCCUS PNEUMONIAE INFECTION

Pneumonia caused by pneumococcus is stopped abruptly when the right antimicrobial agents are administered early. Penicillins V and G, vancomycin, cephalosporins, macrolides and quinolones are some of the antibiotics 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. 

PREVENTION AND CONTROL OF STREPTOCOCCUS PNEUMONIAE

INFECTION

Pneumococcal pneumonia is typically an endemic respiratory infection that occurs occasionally in human populations (who are the main reservoirs of the causative agent). S. pneumoniae is a transient member of the human normal flora; and the organism colonizes the upper respiratory tract (nasopharynx) of both children and healthy adults who harbour pneumococcus asymptomatically. Mortality due to pneumococcal pneumonia is usually high in the elderly, people with impaired natural resistance against the pathogen (e.g., sickle cell patients), and in the debilitated or immunocompromised individuals. Such individuals should be properly vaccinated against the disease and its causative agent since immunization confers a lasting form of protection. The prevention of pneumococcal pneumonia is largely based on immunization using pneumococcal vaccine and the effective treatment of affected individuals. About 90 virulent strains of S. pneumoniae are known, and there exist effective vaccines (Pneumovax 23) for the prevention of a handful of the pneumococcal strains that cause pneumonia in humans (especially the elderly and debilitated people). A seven-valent conjugate vaccine for immunization in children under the age of two years is also available and effective. People at high risk of having respiratory tract infections, those with weakened immunity, the elderly and healthcare personnel’s should be vaccinated against pneumococcus. Pneumovax 23 is believed to help increase host phagocytic action against the capsulated pneumococcus.People should avoid factors that can predispose them to the disease, and treatment should be started early once pneumococcal pneumonia is suspected.    

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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