Streptococcus pneumoniae is a gram-positive bacterium that can be found in both healthy carriers as well as in people that have developed disease. One of the major virulence factors of pneumococci is their polysaccharide capsule. Based on the capsule that surrounds the bacteria, pneumococci are divided into at least 90 different serotypes. Some serotypes seem to be more related to virulence than others.

I have with comparative genome hybridization microarray technique, studied gene differences between 18 epidemiological well-characterised pneumococcal strains with different potential to cause disease. A microarray chip based on two sequenced pneumococcal genomes, R6 and TIGR4, has already been designed. According to Hierarchical clustering, both the serotype and the genetic type as assessed by MLST (sequence type or ST) seem to have impact on the relationship of clinical isolates. Most clinical isolates of the same serotype are clustered together except for serotype 14 isolates that seem to be more divergent. Further more the number of genes that are divergent between clinical isolates compared to R6 and TIGR4 differ from 65 to 289. Preliminary results indicate that although there is diversity among clinical isolates some are more closely related to each other then others. Absent genes seem to be evenly distributed among all 18 clinical isolates tested but hypothetical genes and genes for cell envelope are two groups of role categories that are absent to the largest extent in all isolates.

According to results from microarray analysis, a gene region, spr0112-spr1015- is present in all type 9V isolates and absent in many isolates of serotype 14, 19F and 7F. These results have been confirmed by polymerase chain reaction (PCR).

Conserved genes in a region around the capsule genes have been sequenced to identify marker genes for a capsulular switch between serotype 9V and 14. Preliminary results of the sequencing showed that as much as 750kb might have been transferred in the event of capsular switch.