Assessment of bacterial pathogenesis by analysis of gene expression in thehost

A number of techniques have been developed to assess the expression of micr obial virulence genes within the host (in vivo). These studies have shown t hat bacteria employ a wide variety of mechanisms to coordinately regulate t he expression of these genes during infection. Two tenets have emerged from these studies: bacterial adaptation responses are critical to growth withi n the host, and interactions between microorganisms and the microenvironmen ts of their hosts cannot be revealed from in vitro studies alone. Results t hat support these tenets include (i) the prevalent class of in vivo express ed genes are involved in adaptation to environmental stresses, (ii) pathoge ns recovered from host tissues (versus laboratory growth) are often more re sistant to host killing mechanisms, and (iii) virulence gene expression can differ in the animal compared to laboratory media. Thus, pathogenicity com prises the unique ability to adapt to the varied host milieus encountered a s the infection proceeds.