Virulence factors, Pathogenesis and Clinical manifestations of Streptococcus pneumoniae


Streptococcus pneumoniae (also known as pneumococcus) is a type of bacteria that can cause various infections, such as pneumonia, meningitis, otitis media, sinusitis, and sepsis. It is one of the most common causes of morbidity and mortality worldwide, especially in children under five years old and adults over 65 years old.

Streptococcus pneumoniae are lancet-shaped, gram-positive, facultative anaerobic bacteria that have more than 100 different serotypes based on their capsular polysaccharide antigens . The capsule is an important virulence factor that helps the bacteria evade the host immune system by preventing phagocytosis and complement activation. However, not all serotypes are equally pathogenic or prevalent. Only a few serotypes account for most of the pneumococcal disease cases in different regions and populations.

Streptococcus pneumoniae can be found in the upper respiratory tract (URT) of healthy people, especially children, without causing any symptoms. This is called colonization or carriage . The rate and duration of colonization vary depending on factors such as age, season, vaccination status, and exposure to other people. Colonization is usually harmless and may even confer some protection against invasive disease by stimulating natural immunity. However, colonization can also lead to infection if the bacteria spread to other parts of the body or if the host defenses are compromised by factors such as viral infections, smoking, chronic diseases, or immunosuppression .

The transmission of Streptococcus pneumoniae occurs through respiratory droplets from coughing or sneezing of infected or colonized people. The bacteria can also be transmitted by direct contact with contaminated secretions or surfaces . The risk of transmission is higher in crowded settings such as schools, daycare centers, nursing homes, and hospitals. The incubation period of pneumococcal disease ranges from one to three days.

The pathogenesis of Streptococcus pneumoniae involves several steps: adhesion to the epithelial cells of the URT, invasion of the host cells or tissues, evasion of the host immune response, and damage to the host cells or tissues by various mechanisms. These steps are mediated by different virulence factors that will be discussed in detail in the next section.