USE OF ISOGENIC MUTANTS TO STUDY BACTERIAL VIRULENCE FACTORS

The development of molecular techniques in genetic analysis of bacteri a 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 t hat appropriate conclusions may be derived from experimentation. Just as Koch's postulates are used in defining bacterial etiology of infect ious disease, a molecular version of Koch's postulates is applied to d etermining if particular attributes or functions of bacteria contribut e 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 fun ction resulting in decrease in virulence, and finally that the mutatio n be complemented in trans with only the wild-type allele of the mutat ed gene provided on a recombinant plasmid. The molecular genetic analy sis of the virulence plasmid of Salmonella typhimurium over the past d ecade serves as an example of the application of these rules to a comp lex problem in bacterial pathogenesis. This discussion of molecular Ko ch's postulates and analysis of the S. typhimurium virulence plasmid a re aimed at aiding in the design of a complete and thorough genetic ap proach to the study of bacterial virulence.