Uropathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 282

innate immune system. WaaL ligates O antigen onto the lipid A core subunit
of LPS which reduces detection by TLR4 in the urinary tract, and Alr converts
L-alanine to D-alanine for peptidoglycan synthesis ( Billips et al., 2008 ). Dele-
tion strains of ampG , waaL , or alr were outcompeted by wild-type E. coli NU14
in bladder colonization in the mouse model of infection, demonstrating that
modification of PAMPs is important for uropathogenesis ( Billips et al., 2008 ).
α -Hemolysin (HlyA)
Alpha-hemolysin is a prototypical member of the repeat in toxin (RTX) family
( Welch, 1991 ). While deletion of hlyA did not attenuate UPEC in the mouse
model of ascending UTI, significantly less sloughing of the uroepithelium and
bladder hemorrhage was observed in comparison to wild-type E. coli ( Smith
et al., 2008 ). Epidemiological evidence associates α-hemolysin with highly
virulent strains, as α-hemolysin is more prevalent in pyelonephritis and uro-
sepsis isolates ( Opal et al., 1990 ; Blanco et al., 1996 ; Johnson et al., 2005b ).
α-Hemolysin is secreted by UPEC by the type I mechanism (Chapter 16), and
complexes with LPS which then exploits the CD14/LPS-binding protein to
bring hemolysin into contact with the host cell ( Mansson et al., 2007 ). HlyA
then inserts into the cell membrane of uroepithelial cells, and forms pores,
permeabilizing the cell membrane causing cell death at high concentrations
( Felmlee and Welch, 1988 ; Welch et al., 1992 ; Stanley et al., 1998 ). At sub-
lytic concentrations, α-hemolysin affects signal transduction pathways in the
host cells by inducing an oscillatory change intracellular calcium concentration
( Uhlen et al., 2000 ), which leads to suppression of cytokine release ( Konig and
Konig, 1993 ; Hilbert et al., 2012 ). Further study revealed that insertion of HlyA
into the cell membrane of epithelial cells and macrophages induces activation
of mesotrypsin, a serine protease ( Dhakal and Mulvey, 2012 ). Mesotrypsin
then degrades paxillin, a cytoskeletal scaffold protein and components of the
pro-inflammatory NFκB signaling cascade, thus suppressing the inflammatory
response and disabling macrophages ( Dhakal and Mulvey, 2012 ). α-Hemolysin
integration in the host cell membrane also activates caspases, leading to host
cell death ( Dhakal and Mulvey, 2012 ).
Cytotoxic necrotizing factor-1 (CNF-1)
Cytotoxic necrotizing factor-1 (CNF-1) has been epidemiologically linked
to urovirulent strains of E. coli , with 61% of UTI isolates and only 10% of
fecal isolates encoding cnf-1 ( Yamamoto et al., 1995 ). As mentioned above,
CNF-1 affects host cells by deamidation of small GTPases leading to uroepi-
thelial cell invasion ( Falzano et al., 1993 ; Visvikis et al., 2011 ). Other effects
of CNF-1 include reduction of complement receptor-3-dependent phagocytosis
in monocytes ( Capo et al., 1998 ), and decreased transmigration of PMNs ( Hof-
man et al., 1998 ). CNF-1 activation of RhoA causes an increase in F-actin in
PMNs resulting in actin polymerization and cell spreading. CNF-1 increases