Effects of surface characteristics on the transport of multiple Escherichia coli isolates in large scale columns of quartz sand - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 24

2.4.3 Transport distance dependent sticking efficiency reduction
Sticking efficiencies decreased with increasing transport distance in both DI and AGW, by more
than 1 log unit among the strains and less than 1 log within strains. In addition, in some cases,
Α values greater than 1 were found. The latter is not very strange (Shellenberger et al., 2002;
Morrow et al. 2005; Paramonova et al. 2006), due to the presence of cell surface organelles like
flagella and pili that extend beyond the cell surface. In such case, the aspect ratio used in the
computation of the single-collector contact efficiency might be under estimated.
Our results are consistent with Simoni et al. (1998), Baygents et al. (1998), Redman et al.
(2001a,b), Li et al. (2004) and Tufenkji and Elimelech (2004b, 2005b), who also reported a
reduction of the sticking efficiency with transport distance. In some cases, those variable bacteria
deposition rates were likely caused by variation in the LPS coating surrounding the bacteria
cells, leading to heterogeneous interaction (Simoni et al., 1998), or by variability in surface
charge densities within a bacteria population (Baygents et al.; 1998, van der Mei and Busscher,
2001, Tufenkji and Elimelech, 2004b). Our results indicated that these transport distance
dependent sticking efficiency reductions were caused by the variable presence of motile cells and
Ag43 expression: highly motile cells, expressing the Ag43 adhesin were removed faster than
cells expressing only one of the two or neither one.
2.5 Conclusions
The effect of a number of bacteria properties (Ag43-expression motility, hydrophobicity, outer
surface potential and sphericity) on attachment to quartz grain surfaces in columns of quartz sand
up to 5 m were studied. Our results indicated that sticking efficiencies decreased with increasing
transport distance in both DI and AGW, by more than 1 log unit among the strains and less than
1 log within strains. In addition, we found that Ag43 expression and motility significantly
influenced bacteria attachment, while attachment was not correlated with outer surface potential,
sphericity and cell surface hydrophobicity. Furthermore, we observed that the correlation
between Ag43 expression and sticking efficiency reduced with increased transport distance, and
the same was true for the relation between motility and sticking efficiency. Intra-population and
inter-population heterogeneities exist within and among different E. coli strains, and the
prediction of transport distances based on experimental results with a single strain cannot be
simply extrapolated. The implication of our findings is that less motile bacteria with little or no
antigen expression may travel longer distances once they enter groundwater environments. In
future studies, the possible effect of bacteria surface structures, like fimbriae, pili and surface
proteins on bacteria attachment need to be considered more systematically in order to arrive at
more meaningful inter-population comparisons of the transport behavior of E. coli strains in
aquifers.