Streptococcus pneumoniae (pneumococcus) is a Gram-positive bacterium that is responsible for the majority of community-acquired pneumonia. It is a commensal organism in the human respiratory tract, meaning that it benefits from the human body, without harming it. However, infection by pneumococcus may be dangerous, causing not only pneumonia, but also bronchitis, otitis media, septicemia, and meningitis.
S. pneumoniae is alpha-hemolytic, meaning that it can break down red blood cells through the production of hydrogen peroxide (H2O2). The production of H2O2 by the bacterial infection can also cause damage to DNA, and kill cells within the lungs. Pneumococcal pneumonia causes fever and chills, coughs, difficulty breathing, and chest pain. If the infection spreads to the brain and spinal cord, it can cause pneumococcal meningitis, characterised by a stiff neck, fever, confusion, and headaches.
Pneumococcal infection is responsible for 1-2 million infant deaths worldwide, every year. During influenza epidemics, S. pneumoniae is associated with higher mortality in patients infected with both microorganisms. It is thought that S. pneumoniae and Haemophilius influenzae have a synergistic effect on one another, when infecting the same host.
Fig. 1 Model of Streptococcus pneumoniae bacteria
The genome of S. pneumoniae is a closed, circular DNA structure that contains between 2.0 and 2.1 million base pairs depending on the strain. It has a core set of 1553 genes, plus 154 genes in its virulome, which contribute to virulence and 176 genes that maintain a noninvasive phenotype. Genetic information can vary up to 10% between strains.
Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the surrounding medium. Transformation is a complex developmental process requiring energy and is dependent on expression of numerous genes. In S. pneumoniae at least 23 genes are required for transformation. In order for a bacterium to bind, take up and recombine exogenous DNA into its chromosome it must enter a special physiological state called competence.