Enteropathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 80

1997 ). A binding affinity between bundlin, the major structural subunit of
BFP and N-acetyllactosamine has been established ( Hyland et al., 2008 ).
Although this interaction is weak, it may have a significant effect on LA due
to the potentially large numbers of ligands and receptors involved. More
recent work indicates that N-acetyllactosamine may trigger BFP retraction
in addition to acting as a competitive inhibitor of EPEC binding to the host
receptors ( Hyland et al., 2006 ). EPEC also binds to phosphatidylethanol-
amine, a component of cell membranes ( Foster et al., 1999 ; Barnett et al.,
2000 ; Khursigara et al., 2001 ).
Other surface components contributing to EPEC adherence include intimin,
the EspA filament and other fimbriae and/or pili. Single, double, and triple
mutants of the bfp , espA , and eae genes revealed that while BFP and EspA both
play a role in adherence, BFP plays a more dominant role; atypical EPEC strains
that do not express BFP may use EspA for adherence. While a triple bfpA espAeae
mutant is not adherent to epithelial cells ( Cleary et al., 2004 ). EPEC strains lack-
ing BFP remain able to adhere to pediatric small intestine tissue explants, although
they form smaller colonies that lack the three-dimensional structure ( Hicks et al.,
1998 ). Initial adherence in such strains has been attributed to the production of
E. coli common pilus (ECP) ( Scaletsky et al., 2010a,b ) and in some strains to
the afimbrial adhesion, locus for diffuse adherence (LDA) ( Torres et al., 2007 ).
E. coli strains fitting the definition of EPEC but having non-LA pattern of adhe-
sion, i.e. diffused adherence, have been noted ( Knutton et al., 1991 ; Rodrigues
et al., 1996 ; Beinke et al., 1998 ; Pelayo et al., 1999 ; Scaletsky et al., 1999 ).
This phenotype has been attributed to the omicron subtype of intimin where the
adherence was seen due to an invasive process ( Hernandes et al., 2008 ).
Autoaggregation
When grown in tissue culture medium at 37°C, tEPEC strains aggregate into
large clusters that may contain hundreds or thousands of bacteria ( Figure 4.1 B)
( Vuopio-Varkila and Schoolnik, 1991 ). These autoaggregates are readily vis-
ible under a low-power microscope or even to the naked eye. They are unsta-
ble, dispersing quickly when moved to sub-optimal conditions ( Bieber et al.,
1998 ), with microcolonies in tissue culture forming and then dispersing over
a period of 6 hours ( Knutton et al., 1999 ). Like LA, autoaggregation requires
BFP. Retraction of the pilus fiber causes bundlin pilin subunits to dissociate and
thus the observed dispersion ( Humphries et al., 2010 ). The BfpF protein drives
pilus retraction and thus a bfpF mutant fails to disperse over time ( Knutton
et al., 1999 ).
Attaching and effacing
EPEC has the ability to strikingly alter the surface of the cells to which they
attach. The characteristic phenotype observed during EPEC infection is the for-
mation of A/E lesions ( Moon et al., 1983 ), in which the brush border microvilli