Streptococcus pneumoniae (The pneumococcus) is asymptomatically carried in the nasopharynx of humans. It can also cause invasive diseases such as otitis media, meningitis and pneumonia. Despite the availability of vaccines, it is estimated that pneumococcal disease is responsible for up to 500,000 deaths in children under 5 years of age. One of the major virulence factors of the pneumococcus is the polysaccharide capsule which is the basis of serotyping and protects the organism from the immune system. There are more than 12,500 sequence types of the pneumococcus as determined by multi-locus sequence typing (MLST). The bacterium also shows large genomic diversity in genes coding for surface proteins and other virulence factors.
Serotype 1 strains of the pneumococcus cause a large burden of disease but are rarely found in carriage. Genotypes of serotype 1 have a defined geographic distribution with Sequence Type (ST) 306 found mainly in Europe, ST615 in South America and ST217 in Africa. These three lineages also behave very differently in animal models of infection. ST306 strains have low virulence in animal models and produce a non-lytic version of the pore forming toxin pneumolysin. Expression of non-lytic pneumolysin affects the nature of the inflammatory and immune response to the pneumococcus. The ST615 strain is highly virulent but does not express pneumolysin in vitro and is invasive and able to grow inside cells in tissue culture models. Analysis of these strains at the genomic and transcriptomic level provide clues as to why these strains behave so differently. The ST615 strain demonstrates high level expression homologue of a gene that codes for a protein known to be important in invasion of cells by Gram negative bacteria. This raises the possibly that this protein is involved in the ability of pneumococcus to invade eukaryotic cells.