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5.5.3 Power-law distribution and minimum sticking efficiency
The observed power-law distributions in Α are in agreement with observations made by other
researchers (Lutterodt et al., 2009b, Brown and Abramson, 2006, Tufenkji et al., 2003, Redman
et al., 2001, 2001a,). The large variations in
Α
give rise to the power-law distribution (Lutterodt
et al., 2009b) and may be due to variations in cell surface properties giving rise to differential
affinity of cells for the collector grain surfaces as explained by others (Simoni et al., 1998,
Baygents et al., 1998). The ratio of average cell size to the mean quartz grain size was less than
0.007 which seems too low for straining (Bradford et al., 2007), and the possibility of straining
as a retention mechanism in the set up was therefore considered negligible.
The number of cells exiting the column shows that a substantial number of cells possess both Α
and Α
L
values which are lower than the minimum values measured in our experiments, and
expresses the importance of the extrapolated minimum sticking efficiencies and also the length
of the column used in experiments.
Minimum sticking efficiency values extrapolated from the power law equations were very low
(10
-7
and 10
-8
) and the number of cells (10
6
) possessing this so-called minimum sticking
efficiencies indicates that a substantial fraction of bacterial sub-populations may be transported
over very long transport distances. We have previously (Lutterodt et al., 2009b) observed that
cells possessing the minimum sticking efficiency are non-motile fractions that lack the
expression of Ag43 and other cell properties that may promote bacterial attachment to quartz
grains. The low minimum sticking efficiency values extrapolated compared to values obtained in
a previous work with the two strains (Lutterodt et al., 2009b) may be due to differences in
growth conditions employed in the two experiments. The growth condition employed in this
work (Nutrient broth at 37
o
C) might have caused low expression of cell surface properties that
influences cell attachment to abiotic surfaces, causing low cell retention by the quartz grains. The
two bacteria growth conditions, in the present work and in Lutterodt et al. (2009b) (cow manure
extract at 21
o
C) are comparable to the intestinal and external environmental conditions,
respectively, mimicked by Yang et al. (2006, 2008) who reported significantly higher expression
of cell properties (e.g. Ag43 expression , hydrophobicity and biofilm formation) and higher
retention on bio-barriers (Yang et al., 2008) for a number of
E. coli
strains grown under external
environmental conditions than when grown under intestinal conditions.
5.5.4 Environmental implications
The low values of sticking efficiencies measured, fraction of cells exiting the column and the
extrapolated minimum sticking efficiencies coupled with the number of cells possessing the
minimum sticking efficiencies, have important implications for protecting water abstraction
wells in aquifers. The minimum sticking efficiency gives the Α-values of lower end fractions of
cells in the power-law distribution with non-attaching characteristics. Taking into consideration
factors that affect transport and attenuation of microorganisms in the environment (for e.g.
preferential flow paths, presence of dissolved organic matter and metal oxides), such cells may
be transported over distances much longer than predicted, if results from short column
experiments are extrapolated to the field scale. The low values of extrapolated minimum sticking
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