Streptococcus pneumoniae or pneumococci are gram positive, lancet-shaped (elongated with somewhat pointed end), well-form capsulated bacteria with a size of 0.5 to 1.25 micrometers in diameter, which are usually arranged in pairs(diplococci), but they also seen in short chains or singly.
Structures and their functions:
The name of the structures of Streptococcus Pneumoniae and their important functions are presented in following Table:
|Capsule||Protects the Streptococcus pneumoniae against phagocytosis|
|Cell wall||Gives structural support and maintains the shape of bacteria|
|Cell membrane||Takes part in active transport, energy generation, toxins secretion and synthesis of precursors of cell wall|
|Nucleoid DNA||Genetic material|
|Plasmid DNA||Carry a variety of genes for resistance to antibiotic, toxins and ultraviolet light|
Diagram of Streptococcus Pneumoniae:
(1) Streptococcus Pneumoniae or Pneumococci are gram-positive because their lipid-poor, thick cell wall easily take blue stain during gram staining procedure.
(2) Capsules are virulence factors that limit the ability of phagocytes to engulf the Streptococcus Pneumoniae by preventing the C3b (complement 3b) opsonization of bacterial cells. Capsules may also play a role in the adherence of Streptococcus Pneumoniae to human cells that is an important initial step in causing infection.
(3) There are 85 different serologic types of Streptococcus Pneumoniae are present that are distinguished by the antigenic differences of sugars in the polysaccharide capsule.
(4) Polyvalent pneumococcal vaccine can be produced from purified capsular polysaccharides of 23 serotypes.
(5) In the clinical laboratory, the specific type of Streptococcus Pneumoniae can be identifying by using type specific antiserum that swell capsules. This swelling phenomenon is called quelling reaction.
(6) They are cultured in media that contain blood. On blood agar media, they produce a zone of alpha-hemolysis.
Streptococcus Pneumoniae or pneumococci causes the following diseases
- Otitis media
Humans are the natural hosts for Streptococcus Pneumoniae. Five to fifty percent of health people harbor virulent Streptococcus Pneumoniae in their upper respiratory tract mainly in the oropharynx and transmitted via respiratory droplets. Resistance to pneumococci infection is high in health young people. Usually infection occur when predisposing factors are present.
Factors that lower human resistance and predispose to Streptococcus Pneumoniae infection include
(1) Any cause of brain impairment such as alcohol intoxication, drug intoxication that can depress the cough reflex of a person and increase the chance of aspiration of Streptococcus Pneumoniae containing oropharyngeal secretion into lung.
(2) Viral respiratory tract infections, bronchial obstruction, pooling of mucus and respiratory tract injury caused by irritants predisposes to introduce Streptococcus Pneumoniae into lung by disturbing the mucociliary elevator function of respiratory tract.
(3) Abnormal circulatory dynamics such as lung congestion, heart failure predispose to pneumococcal infection.
(4) Head injury with CSF (cerebrospinal fluid) leakage from the nose predisposes to pneumococcal meningitis.
(5) Splenectomy predisposes to septicaemia.
Pathogenesis (mechanism of disease development):
Streptococcus Pneumoniae produce IgA protease that degrades secretory IgA (one type of body’s antibody) on respiratory mucosa, allowing colonization of organisms. Capsular polysaccharides of bacteria limit the ability of phagocytes to kill them. They multiply in lung tissues and induce inflammation. When the organisms reach alveoli, there is outpouring of fluid, red blood cells and white blood cells, resulting in consolidation (the act of making solid) of the lung.
Pneumonia: A person with pneumococcal pneumonia often present with a sudden high fever, rigors, shivering, cough, rusty color sputum and pleuritic chest pain. Bacteremia occurs in 15-25% of cases with pneumonia.
Meningitis: Symptoms include fever, intense headache, stiff neck and photophobia (intolerance to light).
Sinusitis: Symptoms are fever, facial pain, frontal headache, nasal discharge.
Otitis media: Clinical features include pain in the ear, discharge from the ear cavity, hearing impairment and fever.
Mastoiditis: Presenting symptoms are pain, tenderness, redness and swelling in mastoid region. There may be fever and ear pain.
Bacteremia: Bacteremia is the spread of bacteria in the bloodstream. Presenting complaints may malaise, nausea, abdominal pain, fever, high pulse rate. Without proper treatment, it may quickly progress to sepsis.
Sepsis: It occurs mainly in splenectomized (removal of spleen from body) persons. Here, Streptococcus Pneumoniae spread in the blood and initiating a systemic response that adversely affects the blood flow to vital organs. Symptoms of sepsis are fever, low blood pressure, high pulse rate, rapid respiration etc.
(1) In sputum, Streptococcus Pneumoniae are seen as lancet-shaped diplococci in Gram-stained smears. They can also be identified by using the quelling reactions (swelling of pneumococcal capsule with type-specific antiserum). On culture in blood agar media, Streptococcus Pneumoniae form small α-hemolytic colonies. The colonies are lysed by bile and growth is inhibited by optochin.
(2) Streptococcus Pneumoniae can be detected by blood culture in 15% to 25% of cases.
(3) Pneumococcal capsular polysaccharide can be detected from patients urine by latex agglutination test for diagnosis of pneumococcal pneumoniae and bacteremia .
(4) CSF (cerebrospinal fluid) culture is usally positive in pneumococcal meningitis.
(5) For rapid diagnosis of meningitis, Pneumococcal capsular polysaccharide in CSF can be detected by latex agglutination test.
Most of the Streptococcus Pneumoniae infections are successfully treated with penicillins and erythromycin. Penicillin G is the drug of choice in severe infection, whereas oral penicillin V can be used in mild infection. Antipneumococcal fluoroquinolone such as levofloxacin, sparfloxacin can also be used. In penicillin allergic persons, erythromycin or one of its derivatives such as azithromycin, clarithromycin can be used. In penicillin resistant Streptococcus Pneumoniae, vancomycin is the drug of choice.
Streptococcus Pneumoniae infection can be prevented by pneumococcal vaccine. Two types of pneumococcal vaccine are available. One is pneumococcal conjugate vaccine and another is pneumococcal polysaccharide vaccine. Pneumococcal conjugate vaccine is used in children under the age of 5 years. Vaccination of children reduces the incidence of Streptococcus Pneumoniae infection in adults because children are the main source of bacteria for adults and vaccination reduces the carrier rate in children. Pneumococcal polysaccharide vaccine is used in adults. Oral penicillin is used to prevent pneumococcal infection in hypogammaglobulinemia or splenectomy children.