Pathogenic potential of escherichia coli O26 and sorbitol-fermenting escherichia coli O157:NM
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Date
2010Author
Rosser, Tracy
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Abstract
Verocytotoxin-producing Escherichia coli (VTEC) are important human pathogens
that may cause diarrhoea, haemorrhagic colitis and haemolytic uremic syndrome
(HUS). Worldwide, non-sorbitol-fermenting (NSF) VTEC O157:H7 is the most
common serogroup associated with HUS but several non-O157:H7 serogroups have
emerged as causes of this disease. This research investigated the pathogenic
potential of two non-O157:H7 serogroups: O26 and sorbitol-fermenting (SF)
O157:NM. While VTEC O26 have emerged as a significant cause of HUS in
continental Europe, human infections associated with this pathogen are uncommon in
Scotland and generally only result in simple diarrhoea. The study characterised E.
coli O26 isolates recovered from human infections in Europe and Scotland and
isolates collected from Scottish cattle with the objectives to identify factors which
may allow strains to cause more serious clinical disease and to investigate the
potential of bovine VTEC O26 in Scotland to cause human infection. MLST
analysis of housekeeping genes found little genetic variation in the genomic
‘backbone’ among the vast majority of E. coli O26 isolates. The gene for
verocytotoxin 2 (vtx2) alone was carried by VTEC O26 isolates recovered from
patients in continental Europe but was found in no Scottish human isolate, where the
majority of isolates did not harbour a vtx gene. It was demonstrated that among the
European VTEC O26 human isolates, 67% carried a specific allele within the
promoter region for LEE1 and 87% harboured the tccP2 gene. In contrast, no
Scottish VTEC O26 human isolate carried this allele or the tccP2 gene. The impact
these genotypic characteristics have on the pathogenic potential of a strain remains
uncertain. There were no clear differences in verocytotoxin titres, levels of LEEencoded
protein secretion or levels of adherence to Caco-2 cells between VTEC O26
isolates recovered from human infections of varying severity. However, levels of
LEE-encoded protein secretion from cattle isolates were generally higher than those
from many of the human isolates. The differences in pathogenic potential between
isolates are likely to be due to horizontally acquired DNA, including vtx2 carriage
and the O-island-phage-associated effector protein repertoire. Further work is
required to determine if the differences identified may also impact on shedding levels from cattle and therefore the likelihood of transmission to humans. Since 1988, SF
VTEC O157:NM strains have emerged and have been associated with a higher
incidence of progression to HUS than NSF VTEC O157:H7. This study investigated
bacterial factors that may account for the increased pathogenic potential of SF VTEC
O157:NM. While no evidence of toxin or toxin expression differences between the
two VTEC O157 groups was found, the SF VTEC O157:NM strains adhered at
significantly higher levels to a human colonic cell line. Under the conditions tested,
curli were shown to be the main factor responsible for the increased adherence to
Caco-2 cells. The capacity of SF VTEC O157:NM strains to express curli at 37C
may have relevance to the epidemiology of human infections as curliated strains
could promote higher levels of colonization and inflammation in the human intestine.
In turn this could lead to increased toxin exposure and an increased likelihood of
progression to HUS.