Biology Reference
In-Depth Information
2.1 Introduction
Groundwater systems globally provide 25 to 40% of the world's drinking water (Morris et al.,
2003), and the importance of groundwater can often be attributed to the assumption that, in
general, the resource is free of pathogenic microorganisms (Bhattacharjee et al., 2002). However,
still in many cases, water borne disease outbreaks are caused by the consumption of groundwater
contaminated by pathogenic microorganisms (Macler et al., 2000; Powell et al., 2003). Well-
known sources of contamination are by leakage from septic tanks, unlined pit-latrines, improper
waste disposals, manure, wastewater or sewage sludge (Foppen and Schijven, 2006). One of the
explicit goals set by the United Nations and the international water community is environmental
sustainability, with a target to halve the proportion of people without sustainable access to safe
drinking water and basic sanitation by the year 2015. To achieve this Millenium Development
Goal (MDG) of environmental sustainability, effective management and protection of water
supply sources need to be practiced. Current strategies employed to protect groundwater sources
from contamination rely upon effective natural attenuation of sewage-derived microorganisms
by soils (and rocks) over set back distances (Taylor et al., 2004). While natural processes may
assist in reducing pollution, most biological contaminants can travel through soils and aquifers
until they either enter a water well or are discharged into streams (Corapcioglu and Haridas,
1985).
For a long time, the retention of microorganisms by passage through sand was determined with
the classical colloid filtration theory (CFT; Yao et al., 1971; Schijven, 2001; Tufenkji and
Elimelech, 2004a). The theory is based on the assumption that colloids are retained at an
invariable rate, while deposition decreases log-linear with transport distance. However, recent
research shows that the deposition rate coefficient is not a constant (Albinger et al., 1994;
Baygents, 1998; Simoni et al., 1998; Li et al., 2004; Tufenkji and Elimelech, 2005a,b; Tong and
Johnson, 2007; Foppen et al., 2007). The variation of the deposition rate coefficient has been
attributed to a number of reasons, including geochemical heterogeneity on collector grain
surfaces (Johnson and Elimelech, 1996; Bolster et al., 2001; Loveland et al., 2003; Foppen et al.,
2005), straining (Bradford et al., 2002 and 2003; Bradford and Bettahar, 2005; Foppen et al.,
2007a), and heterogeneity of the colloid population 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., 2007). The variability in bacteria
surface properties has been attributed to variations in lipopolysaccharide (LPS) coating (Simoni
et al.,1998), distribution of the interaction potential within the bio-colloid population (Li et al.,
2004), variations in surface charge densities (Baygents et al.,1998; Tufenkji and Elimelech,
2004b), and differences in energy needed to overcome the energy barrier (Tufenkji and
Elimelech, 2004b).
Three decades ago, a major outer membrane protein termed Antigen 43 (Ag43) was discovered
in
Escherichia coli
(
E. coli
) (Das Gracas de Luna et al., 2008; Owen and Kaback, 1978). Outer
membrane proteins serve a variety of functions essential to survival of Gram-negative bacteria.
Many of these proteins have structural roles or are involved in transport, while others are
important in pathogenesis and have roles in adhesion to host tissue or evasion of the host
immune system (Nikaido, 2003). The outer membrane protein Ag43, encoded by the gene cluster
agn43
, was suggested as critical in determining the adhesive properties of
E. coli
(Henderson et
al., 1997). Recently, Yang et al. (2004) isolated 280
E. coli
strains from a soil, and they found
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