Review of the Food-borne Zoonoses Research ... - ARCHIVE: Defra

In poultry, until recently considered free of O157 infection, hatchlings inoculated orally
with 10 3 cfu were colonised persistently up to and beyond point of lay (20+ weeks of
age). Shedding can be significant (~10 5 cfu g -1 faeces) for this entire period and egg
shells but not egg contents can be contaminated. Eight strains have been tested in the
poultry model and variable colonisation was shown with 2 strains being cleared by 80
days whilst other persisted for 213 days. Intimin was not essential for colonization and no
AE lesions were induced in any section of the gut.
These data point to the role of intimin/Tir in ovines but colonisation of caprines, porcines
and poultry was less dependent upon intimin/Tir as in ovines. Indeed, the data indicated
that long term persistence may be dependent on factors other than intimin and Tir as well
as host factors such as provision of colostrum in the young (negative effect) or prior
colonization by other pathogens (positive effect). To study bacterial factors, we focused
upon those genes identified by analysis of the O157 genome that may contribute to
persistent colonisation of ruminants and sheep in particular. Defined targeted mutants
were constructed (e.g. eae, tir, efa/toxB, waaZ, fliC, lpf, nleD, tccP, ‗O‘ island [O-7, O-36
and O-50] etc) made primarily in E. coli O157:H7 strain NCTC12900 that is a nontoxigenic
strain and other model strains also. Mutants lacking LPS were uniformly
attenutated in all species as might be anticipated. Flagella played a key role in poultry but
no other animals species tested. Of the other factors tested, small contributions to
colonization and persistence were detected for most. To study host factors, we focused
on the general health status and demonstrated that animals pre-inoculated with C.
parvum prior to challenge with O157:H7 developed multifocal attaching and effacing
lesions in the caecum, colon, rectum and at the recto-anal junction, confirmed by
immunohistochemistry to be associated with O157:H7, and shed very high numbers of
O157. Also, neonates deprived of colostrum (sheep and goats) are highly susceptible to
colonization and very high shedding.
From the large animal studies, it was clear that O157 has a tropism for the large intestine
but there was no concrete evidence to support the hypothesis that the terminal rectum or
recto-anal junction was the preferred site of colonisation, as suggested for bovines. What
was interesting was the observation that animals colonised beyond about 28 days p.i.
(and therefore defined as persistent shedders) had O157 organisms throughput the
entire gastro-intestinal tract rather than just the large bowel. The significance of this is
unclear and experiments are in hand now to answer this question.
Another observation from the animal studies was the frequent occurrence of AE lesions
caused of non-O157 bacteria, some of which were identified as belonging to O26, O45,
O86 and O115 serogroups. In a brief survey of E. coli from an abattoir survey, it was
noted that AEEC comprised up to 10% of all the isolates and each was shown to
possess a functional eae (intimin) gene. A selection of these strains has been studied in
detail in the various models in this project and of 33 strains tested in gut loops, all formed
AE lesions. Whether the presence of these AEC interfered with O157 colonisation is
under assessment currently. In one experiment, shedding of O157 after oral challenge by
ovines pre-dosed with O26 showed no differences compared with control animals does
with O157 alone. Surprisingly, O157 suppressed shedding of the resident O26
organisms, an interaction that requires analysis.
Review Summary
These inter-related projects sought to explore the potential for non-bovine food-producing
animals to become colonised with E. coli O157. It was acknowledged that the results
103