Molecular Approaches to Bacterial Vaccines

Recent advances in molecular techniques provided a whole-genome perspective in designing novel and improving existing vaccines against bacterial infections. However, a tremendous amount of information derived from genome sequencing, comparative genomics, microarrays, proteomics, and other large-scale technologies is still not completely utilized to provide a systematic mining of new virulence determinants and prospective vaccine candidates. Therefore, the major task in coming years will be to develop cross-interacting databases which can quickly translate the functional genomic information into potential vaccines. Yet, in many cases, these approaches have already allowed identification of more promising targets than conventional approaches did in the past decades. The bottleneck for moving forward with vaccine technologies resides in the development of high-throughput systems for testing the properties of selected candidate vaccines. However, the more highly developed systems such as bacterial cell surface display are continuing to be improved. Together with the traditionally used outer membrane protein, fimbriae and flagella displays, novel platforms using autotransporters and lipoproteins have brought additional capabilities for the carriers to expose foreign molecules on the cell surface. Particularly important has been the creation of “second generation” vaccine delivery vehicles for mucosal immunization by the addition of codisplayed adhesins for targeting cells to specific immunoreactive sites. Bacterial peptide display has successfully competed with other epitope mapping techniques and, in combination with protein arrays, proven useful in identification of the entire set of antigens of the pathogen targeted by the immune system. In addition, the development of novel delivery systems has significantly advanced by using intracellular pathogens for genetic immunization. Delivery of DNA vaccine plasmid and even translation-competent mRNA directly to the cytosol of the antigen-presenting cells can be efficiently achieved by self-destructing live bacterial carriers. A bacterial ghost delivery system has become popular for heterologous antigen presentation, and also has been tested for DNA vaccination. Yet another technique, the use of protein secretion systems such as types I, III, and IV, had opened new horizons of using “inverted pathogenicity” for vaccine purposes. Further, genetic approaches have improved the area of identification of essential genes, characterization of functions required for growth under certain conditions (e.g., in vivo–induced), and importantly, in screening methods of direct selection of protective candidates. Finally, several novel stabilization systems for the foreign genes expressed in bacterial carriers have been generated and tested with different vaccine carriers.