The influence of Salmonella genomic island 1 on global gene expression of multi-drug resistant Salmonella Typhimurium DT104 in mid-log and early stationary growth phases

A once rare strain, Salmonella Typhimurium DT104 (DT104) acquired multiple drug resistance and has emerged globally and has been associated with increased morbidity and mortality. Increased death rates associated with MDR DT104 may be due to ineffective antibiotic treatments; however,they may also result from a hypervirulent phenotype exhibited by this strain that often harbours Salmonella Genomic Island 1 (SGI1). SGI1 is a 43 kb chromosomal element containing 44 ORFs including genes that confer multi-drug resistance in addition to those with unknown and putative regulatory functions. SGI1-influenced gene expression was assessed in mid-log and early stationary growth phases using microarray analysis with an SGI1 isogenic strain pair of DT104 to determine if it influences genes attributed to virulence and/or increased fitness. In mid-log phase, SGI1 influenced genes involved in O and H antigen variation. A larger portion of the DT104 transcriptome was influenced by SGI1 in early stationary phase including invasion genes activated by and including hilA (SPI1 and SPI4). Up-regulation of invasion genes is supportive evidence that SGI1 is involved in MDR DT104 associated hypervirulence. Future studies should confirm if the invasiveness of MDR DT104 harbouring SGI1 to mucosal cells reflects the invasive gene expression profile when grown to early stationary phase, and the involvement of specific SGI1 ORFs, particularly those with putative regulatory functions, in the up-regulation of hilA.