Transport d’E. coli en milieux poreux complexes et artificiels

Understanding and predicting bacterial transport in porous media has become an important subject of research for its applications in domains such as water source protection and bioremediation (Fontes et al. 1991). Since soils are supposed to act as a filter groundwater has been assumed to be free of pathogenic bacteria, but several contaminations of water sources with micro-organisms were possibly linked to bacterial transport through the unsaturated zone (Krapac et al. 2002). (Unc et Goss 2004). Controlling bacterial fate in soils has also interesting prospects for agronomic issues such as bio control with plant growth promoting strains (Amarger 2002). Concerns are also growing about the use of waste water for irrigation in Mediterranean regions suffering water shortages. Indeed waste water carrying pathogenic strains could contaminate drinking wells when used for irrigation. Thus understanding the mechanisms involved in bacterial displacement and modeling are important for practical applications. During the last two decades several models have been developed to describe micro organism transport in porous media (Chen et Strevett 2003; Johnson et al. 1995; Sen et al. 2005; Tian et al. 2002; Tufenkji et al. 2003). Among all the phenomena involved in bacteria displacement, one important mechanism is the deposition on the solid phase. This has been studied for a long time and the deposition rate has been shown to depend on several parameters among which: flow velocity, solid grain diameter, bacterial dimension, collector efficiency, etc… Correlations have been proposed to relate collector efficiency to elementary mechanisms controlling colloid approach to the solid (Rajagopalan 1976; Tufenkji et Elimelech 2004). In simple porous media made up of glass beads or sand grains, and under favourable conditions (i.e. absence of repulsive electrostatic barrier) the deposition rate has been shown to adequately predict bacterial retention. However, it remains the problem of estimating the collision efficiency under unfavourable conditions (Dong et al. 2002). Many experiments have shown that retention could happen in situation presenting unfavourable conditions (repulsive electrostatic energy barrier) (Tong et Johnson 2006). Experiments have also shown that bacteria were not retained although the conditions seemed favourable (Tong et al. 2005). Hence, the deposition rate is still a subject of study. The release rate of previously retained bacteria is another problem that is being analyzed (Tong et al. 2005). As well, heterogeneity of bacterial This paper presents transport experiments at two ionic strengths, in order to obtain favourable and unfavourable conditions for deposition, with a single bacterial strain. Porous media ranged from glass beads and sand columns to columns repacked with soil aggregates of increasing size and ultimately to undisturbed soil columns. Results are analyzed with a model previously proposed (Li et al. 2005) and with a model accounting for two types of deposition sites. Results show that physical mechanisms can be dominant over chemical mechanisms when the complexity of the porosity patterns increases. This paper presents transport experiments at two ionic strengths, in order to obtain favourable and unfavourable conditions for deposition, with a single bacterial strain. Porous media ranged from glass beads and sand columns to columns repacked with soil aggregates of increasing size and ultimately to undisturbed soil columns. Results are analyzed with a model previously proposed (Li et al. 2005) and with a model accounting for two types of deposition sites. Results show that physical mechanisms can be dominant over chemical mechanisms when the complexity of the porosity patterns increases.

The bacterial strain used in this study was Escherichia coli K12 MG1655 ompR234 (Vidal et al. 1998) (PHL1314) and was kindly given by P. Lejeune (French Institute of Applied Science, Lyon, FRANCE). This strain was selected for its ability to be transported in porous media as described previously (Jacobs et al. 2007). PHL1314 surface tension components were described in (Jacobs et al. 2007). The E. coli strain used in this study was genetically fluorescently labeled using the pDsred-express plasmid (Clonetech, USA). Cells were transformed using the chemical TSS method (Transformation Storage Solution: PEG3350 10%, MgCl2 10mM, MgSO4 10mM, DMSO 5%). The pDsred-express plasmid inserted is carrying the dsred gene witch encodes for the red fluorescent protein from Discosoma sp. reef coral. Dsred maximum excitation wavelength is 556nm and maximum emission is 586nm. The bacterial cells were fluorescently labeled in order to perform flow cytometry enumeration. The pDsred- express plasmid also provides resistance to ampicillin antibiotic.