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attachment efficiency and the number of type I fimbriae present at the surface of the E. coli
strains. However, with the techniques we used we were unable to determine such relation.
The ompC , slp , and surA genes were present in 100% of the strains used, and the importance of
these genes has been demonstrated during the initial stages of biofilm formation (Sauer, 2003;
Otto et al., 2001; Prigent-Combaret et al., 1999; Tabe Eko Niba et al., 2007). However, as is true
for most of the genes studied here, their role in initial attachment of planktonic E . coli cells is
not clear.
The genes iha , flu and bfpA were present in many cases (25-80% of the strains analyzed), but
their presence/absence was not associated with the attachment efficiency. The protein expressed
by the iha gene has been shown to adhere to epithelial cells (Tarr et al., 2000), and to act as a
virulence factor in E. coli O157:H7, but the role of iha in adhering to quartz grains is unknown.
The same is true for the bundle forming pili gene bfpA : the protein expressed by the gene is
known to adhere to biotic cells, but its role in initial attachment to abiotic cells is unknown. The
role of Ag43 in relation to abiotic surfaces has been studied in more detail (Schembri et al.,
2003; Schembri and Klemm, 2001, Yang et al., 2006), but mainly in relation to its increased
importance during the initial stages of biofilm formation, after initial attachment has already
taken place.
Finally, from this research a number of important issues emerge. Firstly, we concluded that
attachment efficiency and LPS composition were associated. But what is then the quantitative
relation between LPS composition and initial attachment of E. coli at the quartz grain surface?
Do strains with similar serotypes have similar breakthrough behavior under all circumstances?
Secondly, we identified the presence of a number of genes, and the presence of the Afa gene was
most associated with the attachment efficiency, but with the techniques used, we were unable to
quantify gene expression. In a next step of this research, the (quantitative) role of proteins
expressed by the genes in initial attachment of E. coli cells should be studied. In addition, knock-
out genes or isogenic mutants to confirm the role of the various surface structures may have to be
used. Thirdly, have we included all relevant surface structures? Still around 38% of the more
than 4200 different proteins encoded by E. coli are of unknown function (Madigan et al., 2009),
and we might well have missed one or more genes involved in the initial attachment of
planktonic E. coli cells. Micro-arrays to allow for the inclusion of other genes, that express
surface structures we have not included in this research, are important and should be utilized.
Finally, except for the UCFL strains, which were harvested from the soil of a pasture (Yang et
al., 2006), all other strains were exclusively isolated from biotic environments. How relevant are
these results from an environmental point of view? Or, conversely, which type of E. coli strains
tend to travel farther, and which ones travel less far? Would it be possible to identify zones or
groups of E. coli in aquifers, according to surface characteristics, type of geology, and type of
hydrochemical environment? These are intriguing questions, which would assist in
characterizing and assessing the fate of E. coli in the subsurface, both as an indicator organism
and as a pathogen.
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