The ecology of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 11

often return with antibiotic-resistant strains they did not appear to have prior to
travel and that these outcomes occur in the absence of antibiotic usage while
traveling ( Rogers et al., 2012 ).
It is very likely that we have a poor understanding of the true infectious dose
required to initiate an infection. Certainly most experimental studies attempting
to establish an E. coli strain in a host use cells grown in vitro and it is likely that
such cells are not in the appropriate physiological state to successfully establish
in the intestine. In Vibrio cholera O1 Inaba El Tor there are data to indicate
that passage through a host results in cells that are, for a time, hyperinfectious
relative to cells that have spent some time in the external environment ( Hartley
et al., 2005 ). A hyperinfectious state has also been observed for the diarrheal
pathogen Citrobacter rodentium ( Bishop et al., 2007 ). Mathematical models
incorporating this hyperinfectious state for Vibrio cholera suggest that this state
plays a significant role in determining the dynamics of the early stages of a
cholera outbreak ( Hartley et al., 2005 ). It may well be true that infectious doses
of many of the intestinal pathotypes are significantly lower than those estimated.
The final component of the basic reproductive value of diarrheal pathogens is
the factors affecting the persistence of the pathogen in the external environment.
Since E. coli is a characteristic component of mammalian feces, Schardinger
proposed in 1892 that its presence in water could indicate the presence of fecal
contamination and therefore the potential presence of enteric pathogens. The
abundance of E. coli in feces, combined with the ease with which E. coli can
be detected and enumerated in contaminated water, led to its development as an
indicator organism for water quality monitoring. Although the indicator concept
and criteria have undergone a number of modifications over the years, E. coli
remains the preferred indicator of fecal contamination for water quality moni-
toring ( EPA, 1986 ; WHO, 2008 ). Indeed, when there are known sources of fecal
contamination, E. coli counts in water are a good predictor of gastrointestinal
disease ( Cabelli et al., 1979, 1982 ).
Savageau (1983) was the first to recognize that the biotic and abiotic condi-
tions of the lower gastrointestinal tract differ markedly from those found in soil,
sediments, and water, and that enteric bacteria such as E. coli must have evolved
mechanisms to cope with both the host and external environments. Although
we have a growing understanding of the molecular and physiological basis of
E. coli 's interaction with the intestinal environment, we do not have a good
understanding of the phenotypic response of a cell when it moves from a host to
the external environment and vice versa. The nature of the cell's response to the
shift between these environments will be a prime determinant of the cell's fate
in the environment it is moving to.
For many years the assumption was that the density of E. coli cells in
the external environment was driven by the balance between the rate at
which cells entered the environment and the rate at which they died, with
the average lifespan of an E. coli cell being about 1 day in water to 5 days in
soil ( Van Donsel et al., 1967 ; Van Donsel and Geldreich, 1971 ; Faust et al.,