Current thoughts on microbial evolution including origin of life
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Pathogenicity Islands and Microbial Evolution of Pathogens
Point mutations, genomic rearrangements, and horizontal gene transfer are essential components in microbial evolution. It is, however, the acquisition and excision of large genomic fragments that rapidly results in the emergence of new pathogenic variants. Phages, plasmids, and PAIs are associated with fast evolutionary movement. It has been proposed that the acquisition of PAIs by some pathogens might be due to defects in DNA-repair genes, which result in higher rates of mutation and recombination in that strain compared with a nonpathogenic strain. Following the transfer of phages, plasmids, or PAIs into new host cells, two genetic processes are important. First, there must be stabilization of the new genetic elements. The high rate of mutations often leading to stop codons in the mobility genes often associated with PAIs might be a mechanism of stabilizing the PAI, conserving the advantageous pathogenic phenotype, and limiting its loss from the strain. Second, there must be optimal expression of the newly acquired DNA. For this to occur the PAI-encoded virulence genes need to be incorporated into the regulatory network of the new host organism. Interestingly, the VPI of V. cholerae carries virulence genes; however, the VPI also carries genes that regulate expression of VPI genes and the genes encoding CT which are found at an independent locus on the V. cholerae chromosome.