Studying bacterial responses to changes in temperature, pH, iron levels, stress and catabolite repression have been useful in identification and characterisation of bacterial virulence dependents. In vitro models of gene expression have previously provided much information on mechanisms controlling bacterial pathogenesis and identification of virulence factors. Bacteria respond to changes in the host systems by modulating gene expression by up-regulation of genes necessary for survival and growth and down-regulation of genes that are no longer required. Thus, genes that appear to be important in in vitro studies may not be important in vivo and vice-versa. A subset of these genes include ivi (in vivo induced) genes, which remain transcriptionally silent during in vitro growth and are only induced during infection. It is difficult to study their regulation using standard methods such as in vitro gene expression. Hence, in vivo expression technology (IVET) was developed to identify genes that are specific to infection.