Evolution of pathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 45

within-species diversity. The major genetic events that underlie the evolution of vir-
ulence are: (a) horizontal transfer of novel genes, and (b) pathoadaptive mutation -
amplification, inactivation, and variation - of existing genes ( Figure 3.2 ).
Horizontal gene transfer (HGT)
This process, also called lateral gene transfer, involves transfer of genetic mate-
rial from one strain to another, i.e. in contrast to vertical parent-progeny transfer
of DNA during reproduction. According to a strict definition, HGT involves
gene exchange between organisms from different species, but a more commonly
used definition is applied to the gene transfer between organisms of the same
species as well. HGT implies addition to the genome of novel genes, not an
exchange by homologous recombination of different copies of the same gene,
even though the mechanism of the gene movement between different strains
could be the same.
HGT genomic elements
Genomic regions of HGT origin are typically carried by three major genetic ele-
ments in the E. coli : plasmids, prophages, and chromosomal islands.
Plasmids
Plasmids are usually circular and self-replicating molecules that co-exist with
chromosomes. These extra-chromosomal elements harbor at least one, if not
multiple, essential virulence determinants in all strains of diarrheagenic E. coli
and Shigella spp. ( Mellmann et al., 2009 ). One pioneering study demonstrated
the pathogenic properties of plasmids in E. coli that causes diarrhea in piglets
( Smith and Linggood, 1971 ). Subsequent studies showed that a major class of
virulence factors encoded by the plasmids are genes conferring resistance to anti-
microbial agents (e.g. cephalosporins, fluoroquinolones, aminoglycosides), toxic
FIGURE 3.2 Different genetic mechanisms of evolution of virulence. Improved adhesion to the
tissue receptors is assumed to increase virulence.