Evolution of pathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 60

Virulence of evolution paradox
The concepts of virulence evolution of the accidental or opportunistic E. coli
pathogens lead to an interesting paradox of the evolutionary mechanisms
involved.
As discussed above, horizontally transferred virulence factors in these
pathogens are most likely of pre-adapted nature, i.e. evolved to play a role in
the commensal lifestyle. Thus, the traits that comprise the major genetic differ-
ences between pathogens and non-pathogens and that are considered to provide
'quantum leaps' in the evolution of virulence, are NOT directly relevant to the
pathogenicity per se.
At the same time, virulence factors that were acquired by gene mutation
could have been selected directly during the infection. Thus, the relatively
minor genetic differences, as small as point mutations, that presumably fine-
tune virulence could be much more relevant to the pathogenicity than the major
genetic events.
Thus, 'Devil is in the details'!
Furthermore, the mobile genetic components that constitute the bulk of the
horizontally transferred gene pool could be mostly of a 'selfish gene' nature,
more like genetic parasitic elements ( Asadulghani et al., 2009 ). Indeed, the
majority of them are clearly of bacteriophage origin. Somewhat similar ecology
could be attributed to the plasmids and chromosomal islands as well, even when
they provide a direct contribution to the fitness of the bacterial organism in
specific environments by encoding some fitness-improving traits. Such fitness
improvement could be only transient from evolutionary perspectives and benefit
primarily the spread of the mobile element in the overall bacterial population
rather than the harboring bacterial host ( Touchon et al., 2009 ). Thus, the main-
tenance of horizontally transferred genes in nature could be driven not by their
adaptive significance for the bacterial organism, but for the genetic elements
themselves. This issue of genetic conflict is only lately becoming a focus of the
evolutionary biology of microorganisms ( van Elsas et al., 2011 ).
POPULATION GENOMICS AND VARIOME OF MICROBIAL
PATHOGENS
The evolution of any species, including E. coli , works on the gene pool of a
species that is represented by the combination of all genes and their variants
that occur in organisms belonging to the species. It is continuously enriched by
the genes being transferred, mutated, sifted, and shuffled under various selective
pressures and by random genetic drift. Structure and dynamics of the gene pool
can be assessed to a certain extent by comparing sequences of one or several
genetic loci across different populations of the same species. Such studies have
defined the field of population genetics that, to date, successfully characterized
the genotypic heterogeneity level and the action of various types of selection