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If we try to establish the main drivers for these attachment efficiency variations, then sphericity,
motility, zeta-potential, and aggregation were not helpful in explaining the inter-strain
attachment variations. Lutterodt et al. (2009a), Bolster et al. (2009), and Walker et al. (2004)
arrived at similar conclusions. However, these authors used a limited number of E. coli strains,
while our sample size was much larger.
Other parameters apparently exerted greater control over the attachment efficiency. LPS
occupies 75% of the surface of the bacterium, and E. coli is estimated to have 10 6 molecules per
cell (Caroff and Karibian, 2003), so possibly one of the parameters exerting great control over
the attachment efficiency could have been the presence of acidic carboxyl and phosphatyl groups
in the LPS in the outer membrane of E. coli . These acidic groups are likely charged with a
negative O-atom (Orskov et al., 1977; Stenutz et al., 2006). The quartz sand in our experiments
was also negatively charged, giving rise to rather unfavorable conditions for attachment, on top
of the fact that all strains already had a negatively zeta-potential. The LPS - attachment
efficiency relation seemed somewhat confounded by other mechanisms, but at present we are
unable to pinpoint these mechanisms. Except for Walker et al. (2004), who focused more on the
role of the charge in the core LPS, a role of the O-antigen lipopolysaccharides in initial
attachment of planktonic E. coli cells in saturated porous media has not been reported.
Of all genes tested, only the presence/absence of Afa was associated (
= p ) with attachment
efficiency. The Afa/Dr adhesin is an essential adhesin in Diffusely Adhering E. coli (DAEC).
DAEC have been identified from their diffuse adherence pattern on cultured epithelial cells, and
they appear to form a heterogeneous group. The first class of DAEC strains includes E. coli
strains that harbor Afa/Dr adhesins (Afa/Dr DAEC). These E. coli strains have been found to be
associated with urinary tract infections (UTIs) (pyelonephritis, cystitis, and asymptomatic
bacteria) and with various enteric infections (Servin, 2005). The role of Afa in the initial
attachment to abiotic surfaces, like the quartz grains we used, has not been studied yet, and
because of its association with the attachment efficiency, we think this role deserves more
attention in the future.
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The genes fimH, ompC , slp and surA were present in all strains analyzed. Of those, only Type I
fimbriae ( fim ) are known to be involved in initial attachment. E. coli mutants lacking fim are
dramatically defective in initial attachment to abiotic surfaces such as polyvinylchloride (PVC)
under stagnant culture conditions in rich culture medium (Pratt an Kolter, 1998, in: Van Houdt
and Michiels, 2005). In another study, a fimA mutant showed defective initial attachment in a
biofilm assay conducted in minimal culture medium on submerged Pyrex slides used as
substratum for biofilm growth in a continuous flow culture bioreactor (Beloin et al., 2004).
Furthermore, increased expression of the fim cluster was observed in biofilms grown in minimal
culture medium in continuous flow-chamber culture (Schembri et al., 2003), and Type 1 fimbriae
appeared to be necessary for early biofilm formation on glass wool in rich culture medium (Ren
et al., 2004). These observations illustrate that Type 1 fimbriae are critical for initial stable cell-
to-surface attachment for several E. coli strains and in a range of different media and biofilm
growing systems (Van Houdt and Michiels, 2005). Although we have no direct evidence of the
presence of the fimbriae, due to its common presence, in our case, we assumed they were
formed, and were likely involved in initial attachment. Possibly there was a relation between
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