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of bacteria attachment in our experiments. In addition, Pratt and Kolter (1998) stated that the role
of the flagellum which promotes bacteria motility in for example biofilm formation, is the
promotion of initial contact. It should be noted that in our experiments we did not observe
significant detachment of motile cells, evidenced by the lack of tailing of the breakthrough
curves at distances between 0.13 m to 0.83 m (
Fig. 2.1
). As such, our observation contrasts with
the view that motility might increase desorption by the liberation of bacteria from attachment
bonds (McCallou et al., 1995).
2.4.2 Hydrophobicity, outer surface potential and cell sphericity
We observed very low
r
-values between hydrophobicity and the
strai
Α
in both DI and AGW. In
addition,
r
-values along transport distance were low, and did not show any pattern. These
observations are in contrast to those of van Loosdrecht et al. (1987a, b) and Jacobs et al. (2007).
On the other hand, the observed non-dependence of cell hydrophobicity on attachment was in
agreement with the findings of Gannon et al. (1991). It should be noted here that the MATH test
is not straightforward for determining the hydrophobic character of bacteria. For example,
Gaboriaud et al. (2006) demonstrated significant contributions of electrostatic interactions in
such tests. However, the influence of pH and ionic strength on the percentage partition into
dodecane is expected to be uniform across the strains, which allowed us to make a comparison of
the various hydrophobicities measured in similar solutions.
Though cell shape has been shown to influence bacteria transport with preferential retention of
elongated cells compared to more spherical cells (Weiss et al., 1995, Dong et al., 2002, Salerno
et al. 2006),
r
-values between sphericity and
strai
Α
and also
Α
were low-negative. Our results
are consistent with Bolster et al. (2006), who also found that bacteria retention and cell sphericity
were not correlated. We hypothesize that this non-dependence of attachment on sphericity in our
case might be due to the narrow range of sphericities (0.40 to 0.57) of the
E. coli
strains we used.
The outer surface potential (OSP) of the strains was not correlated with attachment in both DI
and AGW. This is in contrast to the work of Sharma et al. (1985) and Foppen and Schijven
(2006), who found a strong correlation between surface charge and attachment. Walker et al.
(2005), however, observed differences in bacteria deposition rates though they recorded similar
zeta-potential values. These workers stated that the deposition trend cannot be solely explained
by electrostatic interaction due to the zeta-potential. In addition, de Kerchove and Elimelech
(2005) demonstrated that the application of Ohshima's theory to their experimental data was
inconsistent with known features of the
E. coli
cells they used. They attributed this inconsistency
to chemical and physical inconsistencies associated with the ion-permeable polyelectrolyte layer
at the cell surface. Such inhomogeneities were omitted in the development of Ohshima's theory.
In our case, we concluded that the OSP is a lumped parameter that masked the actual interaction
potential between individual cells and collectors. In studies whereby only one bacteria strain is
used, variations of outer surface potential measurements can successfully give an indication of
the processes involved in initial attachment, but in studies using more bacteria strains, due to the
lumped character of the outer surface potential, the indicative value is completely lost.
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