In Vivo Expression of Bacterial Genes During Human Infections

The pathogenesis of a microbial infection is a complex, dynamic process, constantly evolving within the host. In many instances, the production of virulence determinants is tightly regulated, and their production is modulated in response to the ever-changing environment encountered at the site of infection. It is unlikely that all regulated virulence determinants of a pathogen can be identified in vitro, because it is technically impossible to determine and mimic all of the different environmental stimuli that occur at the site of an infection. This shortcoming hampers our complete understanding of the virulence mechanisms employed by Haemophilus influenzae as well as other human pathogens in general. To overcome this problem, a number of investigators have emphasized the need to study bacterial virulence using organisms engaged in an actual infectious process. Several different methods, such as in vivo expression technology (IVET), signature tagged mutagenesis (STM), differential fluorescence induction (DFI), and microarray analysis, have been created to accomplish this end. Application of these methods to infectious processes caused by various bacterial pathogens has succeeded in identifying collections of genes that define the in vivo lifestyle of a pathogen. These genes can be analyzed and interpreted to gain a deeper understanding of the functions that dictate host specificity, tissue tropism, and disease manifestation (1 –6 ).