The ecology of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 4

may elevate E. coli cell densities. Many different antibiotics have also been shown
to lead to significant increases in E. coli cell density ( Looft and Allen, 2012 ).
However, there are little specific data on the nature of the interactions occurring
between the host, E. coli, and other members of the gut microbiota.
Host effects, gut morphology and dynamics, and the gut microbiota are
not the only factors determining the likelihood of detecting E. coli in a host.
Background levels of contamination are also important. E. coli is more likely
to be recovered from frogs, reptiles, or birds living in association with humans
than in those living in undisturbed habitats such as national parks ( Gordon and
Cowling, 2003 ). The climate where the host lives also appears to be a factor in
determining whether E. coli can be detected in a mammalian host. In Australia,
E. coli is unlikely to be detected in hosts living in the desert and less likely to
be detected in hosts living in the tropics, compared to hosts living in temperate
regions of the country ( Gordon and Cowling, 2003 ).
GENETIC STRUCTURE OF E. COLI
It has long been recognized that there is considerable genetic substructure in
E. coli (reviewed in Chauduri and Henderson, 2012 ). In addition to the well-
known phylo-groups A, B1, B2, and D, there is phylo-group E of which O157:H7
is the best-known member. Other workers recognize the existence of phylo-groups
known as C and F ( Tenaillon et al., 2010 ). Phylo-group C strains are closely
related to phylo-group B1 strains and phylo-group F strains are related to D and
B2 strains. Strains of the cryptic clade I should also be considered a phylo-group
of E. coli ( Luo et al., 2011 ). The relationships among the different phylo-groups
of E. coli are depicted in Figure 1.1 . Although there is a well-established and reli-
able PCR-based method of determining the phylo-group membership of an E. coli
isolate ( Clermont et al., 2000 ; Gordon et al., 2008 ), MLST is the only method,
at present, capable of identifying strains belonging to phylo-groups C, E, and F.
Numerous studies have demonstrated that the distribution of strains belong-
ing to the phylo-groups A, B1, B2, and D is very non-random. For example,
among Australian vertebrates, strains belonging to phylo-group B1 are most
frequently isolated from frogs, reptiles, birds, and carnivorous mammals such
as bats and quolls, whilst B2 strains are rare in such hosts ( Figure 1.2 ). Among
humans living in developed countries such as Australia, the United States, and
Europe, B1 strains are less often encountered than strains belonging to the other
phylo-groups ( Figure 1.3 ). By contrast, phylo-group A and B1 strains appear to
be predominant in humans living in developing countries ( Figure 1.3 ).
Consequently, the morphology and dynamics of the gastrointestinal tract
as well as host diet appear to influence the phylo-group membership of strains
present in a host. There is some experimental evidence to support these conclu-
sions. B2 strains occurred at lower cell densities in rodents fed diets high in
crude fiber as compared to the cell densities they achieved when the crude fiber
concentration of the diet was low ( O'Brien and Gordon, 2011 ). Diet effects may