POPULATION GENETICS OF PATHOGENS
One of the discoveries to come from the application of molecular diagnostic tools to in-fectious diseases is the clonal nature of many infectious diseases. That is, over long peri-ods and large geographic distances, the organisms of a given species isolated from clini-cal samples tend to be so similar in chromosomal genetic makeup (and in their plasmid profiles) that one is forced to envision that a clone of bacteria descended from a relatively recent common ancestor is responsible for all or most of the disease incidence. This evi-dence comes partly from studies of plasmid profiles, but mostly it is a conclusion drawn by examining the specific alleles of various genes present in a population of cells using the technique of multilocus enzyme electrophoresis. Differences in electrophoretic mi-gration are used to detect subtle differences in amino acid sequence in a battery of two to three dozen different enzymes. The results have been striking. For example, isolates of Bordetella pertussis from the United States represent a single clone, whereas in Japanthere is a slightly different clone. Another study has determined that only 11 multilocus genotypes (clones) of Neisseria meningitidis have been responsible for the major epi-demics of serogroup A organisms worldwide over the past 60 years. These discoveries provide an entirely new method for study of the epidemiology of infectious disease.
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