Spot the difference: applications of subtractive hybridisation to the study of bacterial pathogens.

Comparison of DNA from virulent strains of bacterial pathogens with DNA from less virulent or avirulent close relatives allows the identification of those genomic regions that are present only in virulent strains. Such regions are often associated with pathogenicity islands (PIs) and their characterisation can lead to a greater understanding of the pathogenesis of infectious diseases. There is now a large database of bacterial genomic sequences that provides useful reference information with which to compare the genomes of strains that exhibit variations in virulence or host preferences. Subtractive hybridisation (SH) and its sister method, suppression subtractive hybridisation (SSH), are techniques designed to identify those regions present in one genome but absent from another. The application of these techniques has led to the identification of PIs, mobile genetic elements and variations in virulence gene expression in a range of bacterial pathogens.