USE OF ISOGENIC MUTANTS TO STUDY BACTERIAL VIRULENCE FACTORS

The development of molecular techniques in genetic analysis of bacteria has enabled detailed dissection of complex problems in microbiology. Among these is bacterial pathogenesis. However, use of the new tools must still be coupled with adherence to rules of genetic analysis so that appropriate conclusions may be derived from experimentation. Just as Koch's postulates are used in defining bacterial etiology of infectious disease, a molecular version of Koch's postulates is applied to determining if particular attributes or functions of bacteria contribute to virulence. The molecular version of Koch's postulates states that an epidemiological association exists between presence of a trait and virulence, that a specific mutation be constructed to inhibit the function resulting in decrease in virulence, and finally that the mutation be complemented in trans with only the wild-type allele of the mutated gene provided on a recombinant plasmid. The molecular genetic analysis of the virulence plasmid of Salmonella typhimurium over the past decade serves as an example of the application of these rules to a complex problem in bacterial pathogenesis. This discussion of molecular Koch's postulates and analysis of the S. typhimurium virulence plasmid are aimed at aiding in the design of a complete and thorough genetic approach to the study of bacterial virulence.