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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Host-Parasite Relationships

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Toward a Genetic and Molecular Definition of Pathogenicity

The classic investigation of pathogenicity has been based on linking natural disease in humans with experimental infection produced by the same organism.

TOWARD A GENETIC AND MOLECULAR DEFINITION OF PATHOGENICITY

The classic investigation of pathogenicity has been based on linking natural disease in humans with experimental infection produced by the same organism. The analysis of bacte-rial virulence determinants usually was the result of the comparative analysis of different clinical isolates of the same species that were either virulent or avirulent in a particular model system. This led to speculation about the potential role of a number of microbial traits as virulence determinants.

This comparative approach now has given way to mutational analysis within a single or limited number of strains of a pathogenic species. The goal is to obtain a single, defined genetic change that alters a single virulence property and affects the pathogenesis of infec-tion or the ability of the organism to cause pathology in an appropriate model system. The advances in microbial genetics, DNA biochemistry, and molecular biology have made it possible to apply a kind of molecular Koch’s postulates to the analysis of virulence traits.

1.   The phenotype or property under investigation should be associated significantly more often with pathogenic strains of a species than with nonpathogenic strains.

 

2.   Specific inactivation of the gene or genes of interest associated with the suspected vir-ulence trait should lead to a measurable decrease in virulence.

 

3.   Restoration of pathogenicity or full virulence should accompany replacement of the mutated allele with the original wild-type gene.

 

This simplistic goal is not always possible because it is dependent on a suitable infec-tion model in which to test a microorganism. The ideal model can be infected by a natural route using numbers analogous to those seen in human infection and can duplicate the relevant pathology observed in the natural host. Except for other primates, such models do not exist for pathogens that are restricted to humans. For example, it is still difficult to assess the role of IgA1 protease in the pathogenicity of Neisseria gonorrhoeae, because the enzyme works only on human IgA1 and the microorganism is an exclusive human pathogen.

    Despite these technical limitations, there has been a revolution over the past decade in understanding of the basic pathogenic mechanisms and how microbes bring about infec-tion and disease. The use of transgenic animals, reconstituted human immune systems in rodents, and the extension of cell and organ culture methods to the study of infectious agents will lead to greater understanding of the pathogenesis of infectious diseases. In parallel, new methods to visualize living microbes in tissue and to monitor genetic activ-ity through “reporter molecules” will permit the monitoring of microbes in infected tissue in real time. The full genomic sequence of most pathogenic microbial species will be completed within the coming decade. This information, coupled with contemporary tech-nology of DNA arrays and the parallel knowledge about the human genome, soon will al-low examination of the expression of every bacterial gene and a representative expression of host genes in both experimental infection models and in samples obtained from in-fected patients. This knowledge will continue to impact how infectious diseases are diag-nosed, treated and prevented in the not-too-distant future.

 

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