Molecular Characterization of Bacterial Virulence Factors and the Consequences for Vaccine Design The 1988 Fleming Lecture

Abstract

The pathogenesis of bacterial infections involves a complex series of interactions between the pathogenic micro-organisms and the host. The outcome of an infection will depend on a combination of factors including the virulence of the bacterial pathogen, the immune status of the host and the innate resistance of the host. In recent years we have learned to apply molecular techniques to the study of bacterial virulence. Molecular approaches allow us to identify and characterize at a structural, biochemical and immunological level particular bacterial virulence factors. The information we gain from such studies can be used together with in vivo and in vitro models to investigate the immune response to individual bacterial antigens. In the future these studies may lead to the development of novel immunoprophylactic, chemotherapeutic and diagnostic reagents. In this lecture I will present a personalized review of recent molecular studies on bacterial pathogens with particular reference to the potential for vaccine development. Molecular studies on bacterial virulence determinantsArguably one of the most important early projects using genetic approaches to study bacterial virulence was the work of H. Williams Smith and colleagues on the enterotoxigenic Escherichia coli of animals (Smith & Linggood, 1971). The approaches taken by Smith have served as a model for many subsequent investigations by other workers. Smith used simple genetic crosses to transfer putative virulence determinants including those for haemolysins, enterotoxins and adhesion fimbriae between E. coli strains to create isogenic pairs of strains differing in one or other factor. Of great importance was the fact that he used the relevant target host species for his investigations such as pigs or cattle and did not substitute these with less relevant, but more convenient species. His approach was made easier because the genes under study, e.g. K88 or K99 adhesion antigens, the heat-stable (ST) and heat-labile (LT) enterotoxins or haemolysins, were naturally encoded on plasmids. Now we are able to readily transfer naturally chromosomally encoded genes between species using in vitro DNA manipulation supported by classical genetic techniques. Smith's work highlighted the important role of adhesion antigens in promoting colonization of the small intestine by pathogenic bacteria and the role of the enterotoxins in inducing diarrhoea (Smith & Linggood, 197 1).Probably the first person to apply molecular approaches and gene cloning to the study of bacterial virulence was Stanley Falkow, whose group began cloning the virulence factors characterized so effectively by Smith. Falkow's cloning work on E. coli LT and ST played an important role in the subsequent molecular characterization of these important toxins (So et al., 1976, 1978). While in Falkow's laboratory I began a study on the molecular characterization of E. coli adhesion antigen determinants. K88 are the adhesion fimbriae most commonly associated with porcine enterotoxigenic E. coli. K88ac was cloned o...