Using a technique called TraDIS, for transposon directed insertion-site sequencing, scientists at the Wellcome Trust Sanger Institute led work that mapped 370,000 unique transposon insertion sites to the Salmonella enterica serovar Typhi chromosome. The density and resolution -- "an average of one every 13 base pairs," they say -- has allowed them to assay genome-wide for "essential genes" and create a list of candidates.