The ecology of Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 9

population requires that every infection will cause at least one new infection,
that is, the basic reproductive value R 0 must be >1. The basic reproductive value
R 0 of a diarrheal pathogen has three components; 1, the average number of cells
shed per infected host over the period of infection; 2, new hosts are infected
at a rate that depends on the average number of cells ingested per host and the
number of cells required to initiate an infection; and 3, the lifespan of cells in
the external environment.
In developing countries, it seems reasonable to assume that most of the
diarrheal pathotypes of E. coli can be maintained in the human population in
the absence of any other host. Asymptomatic carriage coupled with low levels
of sanitation and a lack of clean water, are probably sufficient to maintain these
diseases even if they are human-specific. The maintenance of EHEC strains
can probably be attributed to their asymptomatic carriage in ruminants and
subsequent distribution via the food chain to humans, although there are some
doubts concerning the ability of cattle to be the sole maintenance host ( Besser
et al., 2011 ).
The maintenance of human-specific diarrheal pathotypes in developed coun-
tries is more difficult to understand. The number of hosts shedding the pathogen
will represent only a small subset of the total population. Acquired immunity
and age-related resistance are the most important determinants of the number
of hosts infected by a particular pathotype ( Nataro and Kaper, 1998 ). Acquired
immunity is thought to be important in determining the nature of the susceptible
population (young children) in regions where ETEC is endemic, while it is age-
related physiological changes that have been suggested as the dominant factor
in EPEC infections being restricted to children and the young of other species.
By contrast, there is no evidence to suggest that acquired immunity influences
the establishment of commensal E. coli in the human gut. Host age effects may
occur, as there are some data to suggest that strains belonging to phlyo-group
B1 are more likely to be recovered from children than from older adults living
in developed countries ( Figure 1.4 ).
Although the data for commensal isolates are far from adequate, the evi-
dence indicates that a commensal E. coli strain can persist in an individual host
for periods of months to years ( Caugant et al., 1981 ; Clermont et al., 2008 ;
Reeves et al., 2011 ). Throughout this period commensal strains will shed cells
at the rate of about 10 8 cells per day, although among-sample variation in the
number of cells recovered per gram of feces may vary by more than 100-fold
for the same individual ( McOrist et al., 2005 ). By comparison, E. coli -induced
diarrheal disease generally lasts for less than 2 weeks, although some shedding
of cells may continue after symptoms have passed ( Nataro and Kaper, 1998 ).
However, the number of cells diarrheal pathotypes shed per day is generally
much larger than for commensal strains.
In developed countries asymptomatic carriage appears to be rare and the
genes associated with the various pathotypes are seldom detected. For exam-
ple, in a survey of 489 human fecal samples collected in Melbourne, Australia,