Streptococcus pneumoniae
Introduction
Streptococcus pneumoniae is a gram-positive, alpha-hemolytic, or beta-hemolytic bacterium of the genus Streptococcus. A significant human pathogenic bacterium, S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century, and is the subject of many humoral immunity studies.
Morphology
S. pneumoniae are encapsulated diplococci bacteria. The bacterial cell is ovoid, and typically appears in pairs, or diplococci, but can also appear singly and in short chains. As a Gram-positive bacterium, S. pneumoniae is not decolorized by alcohol after being stained, allowing it to retain a dark blue or purple color. The encapsulated, Gram-positive coccoid bacteria have a distinctive morphology that appears lancet-shaped (pointed like a lance tip), as seen through a microscope.
Pathogenesis
The pathogenesis of S. pneumoniae stems from its ability to avoid host defenses. The organism's polysaccharide capsule allows it to resist phagocytosis, and it can also evade the immune system by invading the bloodstream and organs. The organism also produces toxins that can damage host tissues. Pneumolysin, an enzyme produced by the bacterium, can lyse cells and activate the classical complement pathway. Autolysin, another enzyme, is involved in bacterial cell lysis and the release of bacterial DNA.
Clinical Manifestations
S. pneumoniae is the most common cause of bacterial pneumonia and otitis media, and it also causes sinusitis and meningitis. In pneumonia, infection by S. pneumoniae is characterized by chills, fever, chest pain, and a productive cough. In otitis media and sinusitis, symptoms include pain and a purulent discharge. Meningitis caused by S. pneumoniae is associated with fever, headache, photophobia, and signs of meningeal irritation.
Diagnosis
Diagnosis of an infection caused by S. pneumoniae involves culturing the bacteria from a patient sample, such as sputum or blood. The organism is identified by its optochin sensitivity, bile solubility, and alpha-hemolytic properties on a blood agar plate. Serotyping can be performed to determine the specific capsular polysaccharide type.
Treatment and Prevention
Treatment of S. pneumoniae infections is typically achieved with antibiotics. However, the emergence of antibiotic-resistant strains has complicated treatment. Vaccines that protect against some of the most dangerous types of S. pneumoniae have been developed and are recommended for all children and adults at increased risk of disease.
Epidemiology
S. pneumoniae is found worldwide, and it is a leading cause of illness and death in many developing countries. The bacterium primarily affects the very young and the elderly, and it is a major cause of pneumonia, sepsis, and meningitis.