Streptococcus pneumoniae, commonly known as pneumococcus, is a widespread bacterium often found residing harmlessly in the human respiratory tract. Despite its frequent presence, this microorganism is a significant cause of various infections globally. Understanding its characteristics provides insight into its ability to cause disease and its impact on public health.
Fundamental Biological Traits
Streptococcus pneumoniae exhibits distinct physical and metabolic traits that define its existence. This bacterium is Gram-positive, retaining the crystal violet stain due to a thick peptidoglycan layer in its cell wall. Microscopically, it appears as spherical or ovoid cells arranged in pairs, known as diplococci, or sometimes in short chains.
The bacterium is a facultative anaerobe, able to grow with or without oxygen. Its primary habitat is the human nasopharynx, where it can live without causing symptoms in many individuals. Despite its ability to move within the body, Streptococcus pneumoniae is non-motile.
Defining Virulence Attributes
The ability of Streptococcus pneumoniae to cause illness is closely tied to specific features known as virulence attributes. Foremost among these is the polysaccharide capsule, a thick outer layer. This capsule acts as a shield, preventing immune cells from engulfing and destroying the bacterium. Over 100 different capsular types have been identified.
Beyond the capsule, Streptococcus pneumoniae produces pneumolysin, a protein toxin that creates pores in host cell membranes, contributing to tissue damage and inflammation during infection. Adhesins are surface proteins that enable the bacterium to attach to host cells. They are important for initial colonization and subsequent infection.
The bacterium also produces enzymes that contribute to its survival and spread. For example, some strains produce proteases that break down antibodies, such as immunoglobulin A (IgA), a defense mechanism in mucous membranes. These enzymes help the bacterium evade local immune responses, allowing it to establish infection.
How Characteristics Lead to Illness
The characteristics of Streptococcus pneumoniae directly influence its capacity to cause a range of illnesses. The polysaccharide capsule’s role in evading the host’s immune system is a primary factor enabling invasive diseases. When the bacterium bypasses the body’s defenses due to its protective capsule, it can spread from the nasopharynx to normally sterile sites. This can lead to severe conditions such as pneumonia (an infection of the lungs), meningitis (an inflammation of the membranes surrounding the brain and spinal cord), or sepsis (a life-threatening systemic inflammatory response if the encapsulated bacteria enter the bloodstream).
The bacterium’s ability to colonize the nasopharynx, aided by its adhesins, allows for potential spread. From this initial site, it can migrate to adjacent areas, leading to localized infections.
For instance, spread from the nasopharynx into the middle ear through the Eustachian tube can result in otitis media, a common ear infection. Similarly, movement into the sinuses can cause sinusitis. The combined action of its capsule, toxins like pneumolysin, and tissue-damaging enzymes allows the bacterium to overwhelm local defenses and establish infections in various body sites.
Identification in the Laboratory
Laboratory identification of Streptococcus pneumoniae uses its unique characteristics. When observed under a microscope after Gram staining, the bacterium appears as Gram-positive diplococci. Culturing the organism requires specific growth conditions, as it is a fastidious bacterium that thrives on blood agar plates in a carbon dioxide-enriched atmosphere.
Key biochemical tests also aid identification. Streptococcus pneumoniae is sensitive to optochin, distinguishing it from other alpha-hemolytic streptococci. Additionally, it exhibits bile solubility, meaning its cells lyse when exposed to bile salts. These two tests are commonly used to confirm its presence.
Historically, the Quellung reaction, which causes the capsule to swell when mixed with specific antibodies, was used for serotyping. This method visually confirms the presence and type of the polysaccharide capsule. Modern laboratories also use molecular methods for rapid and precise identification and serotyping.