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over the
E. coli
mass in the influent suspension ranged between 0.001 to 0.36, indicating that
0.1-36% of the initial bacteria mass must have had an Α value less than the lowest Α values
determined for the most distant column segments. For the AGW experiments, removal of the
E.
coli
mass was complete, while for one strain (UCFL-94), still 20% of the bacteria cells of the
influent suspension had a sticking efficiency less than 10
-3
. We showed that the power-law
distribution described best the variations of Α-values within the strains in both solutions (DI and
AGW), although in AGW, the exponential distribution of Α values was equally well capable of
describing the distributions of Α values. Minimum sticking efficiencies, tentatively defined as
the sticking efficiency belonging to a retained bacteria fraction of 0.001% of the original bacteria
mass (total number of cells) flowing into the column (
F
= ), and coinciding with a 99.999%
reduction of the original bacteria mass, were extrapolated from the fitted power law distributions.
Minimum sticking efficiency values ranged from as low as 10
-9
for UCFL-94 to 10
-2
for UCFL-
348 in the DI water experiments, and from 10
-6
for UCFL-94 to
5
10
for UCFL-348 in the AGW
≥
1
experiments.
4.4.1 Sticking efficiency variations within and between E. coli strains
In both DI and AGW Α varied from segment to segment, indicating differences in interactions
between cells and the quartz grains. Large Α values in the top segments of the column for all
strains were attributed to removal of a stickier fraction of the population relative to other cells
within the strains (Albinger et al, 1994; Baygents et al., 1998.; Simoni et al., 1998; Li et al.,
2004; Foppen et al., 2007a,b). The good fit of the power law for all AGW experiments and three
of the DI experiments (UCFL-71, 131 and 263) could be attributed to the comparatively high
retention at the column inlet where stickier fractions were removed resulting in a wide variation
of all
Α
-values.
In AGW, the electrostatic repulsive barrier was reduced resulting in an increased attachment and
fuelling significant attachment in the first segment and contributed to an even wider distribution
in all Α-values compared to the DI experiments. We think that the differences in Α-values can
be attributed to heterogeneity in cell population within the strains, due to variability in surface
properties (Albinger et al.,1994; Baygents et al., 1998.; Simoni et al., 1998; Li et al., 2004;
Tufenkji and Elimelech, 2005a,b; Tong and Johnson, 2007; Foppen et al., 2007a,b; Lutterodt et
al., 2009a). From Yang et al. (2004) and Yang (2005), we know that the strains we used indeed
have (surface) characteristics variations related to variations in zeta-potential, motility,
hydrophobicity, and expression of an outer-membrane protein produced by the so-called antigen
43. This protein is thought to enhance the initial attachment of
E. coli
cells (Henderson et al.,
1997), and was confirmed by our earlier work (Lutterodt et al., 2009a), in which we
demonstrated that
E. coli
strains having the protein expressed at the outer surface membrane
were stickier than strains without Ag43 protein
.
In that work we observed a reduction in the
correlation of Ag43 expression and sticking efficiency along transport distance and the same
observation was made for the relation between motility and sticking efficiency indicating a
possibility of preferential removal of motile cells expressing the Ag43 adhesin. It can therefore
be concluded that within a bacteria population, non-motile cells that neither express the Ag43
adhesin nor other surface characteristics that may facilitate cell adherence to the pure quartz
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