The capacity to quickly and efficiently regulate gene expres- sion has helped bacteria to colonize virtually every available niche in the biosphere, including dynamic and extreme ones. In accordance with this, bacterial populations must be ever ready to either take advantage of a favorable change or hunker down when the going gets tough. This proficiency is nicely demon- strated by infection of humans by facultative pathogens, for which up-regulation of genes necessary for survival and growth and down-regulation of genes deleterious to infectivity must occur on cue. To better understand this transition from ex vivo to in vivo conditions and to further our understanding of pathogenesis, it is necessary to identify genes that are specific to infection. Toward this end, in vivo expression technology (IVET) was developed (27). The purpose of this short review is to update the reader on the many variations of IVET that have been developed, to discuss nuances of each method that may be helpful to investigators embarking on studies using this technology, and to discuss offshoot technologies of IVET as tools for studying regulation of virulence genes. The many individual microbial virulence factors that have been identified using IVET are not reviewed here but have been recently reviewed by Mahan et al. (26). Because IVET is but one of several methods that can be used to screen for virulence genes induced during infection of cultured cells but is the only es- tablished method for accomplishing the same feat within in- fected animals, discussion herein is limited to reported uses of IVET in live animal hosts.
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