An investigation of interactions between bacteria & soil

In Melbourne, Australia, the majority of potable water supplied to the consumer does not undergo filtration. Although this regime is supported by the high quality of the source catchments and the historical integrity of the supply, it is nevertheless important to assess whether particulate matter able to enter the distribution system poses some potential risk. As part of that process, this project investigated how readily bacteria are able to associate with, and be retained by, soil collected from a major holding reservoir. The nature of these interactions is considered of particular interest given reports that surface-associated microbes may exhibit a reduced susceptibility to disinfection. Interactions between bacteria and soil-derived collector media were therefore quantified under various conditions through packed-bed column and batch deposition assays, with the data obtained analysed in terms of measured surface characteristics and interaction modelling. From that, a strong correlation was found between deposition and forces subsumed by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Specifically, it was consistently noted that when electrostatic double layer (EDL) repulsion was reduced, deposition increased. Although reasonable discrimination between the relative retention of bacterial strains was achieved through such an approach, the relationship between surface characteristics and deposition was less clear for the different collector media. That is, measurable physicochemical properties alone could not account for the divergent ability of the different samples to retain bacteria. This led to the suggestion that retention within the studied systems results from a number of intimately linked mechanisms; postulated to be both physicochemical and mechanical in nature. It was thus of interest to determine whether the relative influence of these mechanisms could be discerned. Initially, column assay data indicated that (collector-dependent) retention is likely due to some combination of charge heterogeneities (arising from both morphology and surface chemistry) as well as the provision of stagnation flow points and/or sites for mechanical (entrapmenttype) removal. Batch deposition studies -- which effectively preclude retention by straining or wedging -- meanwhile, confirmed that substantial physicochemical-type deposition occurs. Thus, it was proposed that initial bacteria-soil media interactions are mediated largely by EDL interactions, with the persistence of deposition then a function of the surface properties of a given bacterium-collector system as well as the prevailing conditions (e.g., solution chemistry, hydrodynamics). In terms of Melbourne’s drinking water supply, the data obtained provides evidence that bacteria are able to associate with soil media gathered from a primary holding reservoir and that persistent retention may result. This work thus provides the foundation for further research that will assist in quantifying any risk resulting from this. It also furthers the broader understanding of the mechanisms governing microbe-soil interactions by describing bacterial deposition to a heterogeneous, environmentally derived substrate.