Summary, conclusions and recommendations - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 116

were obtained by extrapolations. The minimum sticking efficiency gives the Α-values of
lower end fractions of cells in the power-law distribution with non-attaching characteristics.
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 efficiencies compared with measured values of sticking efficiencies over
the total transport distances and column segments ( Table 8.1 ) makes the minimum sticking
efficiency a valuable tool in delineating well-head protection areas in real-world scenarios .
8.3 Effect of cell characteristics on the transport/attachment of E. coli cells
Results from the experiments revealed that none of the studied phenotypic characteristics
(motility, hydrophobicity, cell aggregation, cell size, cell surface charge i.e. zeta potential or
outer surface potential) promoted E. coli retention on the quartz grains in solutions of high
ionic strength. Though cell motility and Antigen-43 (Ag43) expression significantly
influenced E. coli attachment to quartz grains over relatively short transport distances in low
ionic strength solutions ( chapter 2 ), under a more relevant groundwater chemistry (solution of
magnesium sulphate and calcium chloride) in chapters 3 and 7 , results indicated a lack of
correlation between bacteria attachment efficiency and the two E. coli characteristics
(motility and Ag43-expression).
Though the transport and E. coli characterization experiments in chapters 3 and 7 ,
respectively, revealed a non-significant correlation and a non-correlation of E. coli serogroup
with transport characteristics, 58% of all spring E. coli isolates that were serotyped belonged
to the same serogroup O21 (serotype O21:H7). It is therefore speculated that the specific
serotype O21:H7 may possess certain characteristics that promote their selective transport
through the aquifers in the Kampala area in Uganda. The homogeneity in transport
characteristics of the spring isolates together with high percentage of strains belonging to the
same serotype that were transported through the springs of Kampala and the two log unit
variation among E. coli isolates in chapter 3 (from soils and zoo animals) indicated that, for
various bacteria strains used in laboratory controlled experiments, the source of isolation may
have a significant influence on the transport characteristcs especially when travel distances
are short (<10 cm).
8.4 Recommendations for future research
Although this research unraveled the distribution in sticking efficiency at large inter-transport
distances, and therefore added to the evidence that the classical colloid filtration theory is
over-simplified when applied to predict bio-colloid transport distances in saturated porous
media, the effect of cell properties on the transport and attachment of bacteria strains to
quartz sand (a predominant aquifer media) still remains a mystery. This is in spite of the fact
that the variability in transport and attachment characteristics within and among different E.
coli strains have been attributed to different cell surface properties or a complex combination
of cell surface organelles. It is therefore important for future research to focus on cell surface
structures known to be involved in initial attachment to host tissues and/or abiotic surfaces.
This will involve the identification of the relative contribution of various cell surface