This chapter aims to provide a better understanding of the bacterial cell attachment and biofilm formation on the mineral surfaces, which would result in improving our knowledge about: the interfacial forces governing the bacterial cell attachment, predicting trends of the biofilm formation and consequently biodegradation rates, and the contaminant’s fate in the diverse geological media (Pouran HM. Studying molecular and nanoscale interactions at metal oxide surfaces and their effects on bacterial adhesion, 2009). In both aqueous and terrestrial environments, bacterial cells tend to be attached to a surface and form biofilm. If they are associated to, e.g., a mineral surface, bacterial cells would remain in a more stable microenvironment instead of being removed by the water shear stress. Even the bacterial planktonic phase can be considered as a mechanism for translocation from one surface to the other rather than a prime lifestyle (Watnick and Kolter 2000; Young 2006). The biofilm formation, which completely covers the surface, initially begins by the adhesion of a small quantity of cells (Vadillo-rodri et al. 2006; Pouran et al. 2017). Among the different indigenous microbial species in the contaminated environments, some are capable of degrading pollutants and participating in the environmental remediation process. The bioremediation process of the contaminated soils and waters is often considered a promising low risk management tool. Even when the contamination poses an imminent threat and other approaches are essential, bioremediation often is a viable secondary strategy for the site maintenance (Haws et al. 2006; Pouran et al. 2017). Natural environments are dynamic and complex systems; therefore, characterization and identifying the underlying processes governing the contaminant’s fate are not easy. Examples of the natural environments heterogeneity are the diverse physicochemical properties of the soils and aquifers matrices (Stumm and Morgan 1996). As the soils and sediments are the prime surfaces for the bacterial cell attachment in most natural environments, elucidation of the surface properties of these constituents and their role in initiating cell adhesion and biofilm formation are of the key importance in understanding the bioremediation process. In fact, the cell-mineral interface reactions not only influence the biodegradation process but many natural phenomena are affected by them. Understanding role of physicochemical interactions at the bacterial cells and minerals interface in the cell adhesion (as well as biofilm formation, development, and behavior) is essential for planning effective bioremediation techniques. It could potentially help us to predict the contaminants’ fate, and trends of the biodegradation rates in different environments. Consequently, the improved knowledge of the cell-mineral interface enable us to design and apply more sophisticated bioremediation techniques as a viable approach towards tackling the soil and water environmental pollution problems. Figure 1 schematically represents an aquifer and biofilm formation on some of the most abundant minerals in the environment, iron and aluminum oxides. It also indicates some the major effects of cell-mineral interface interactions on different environmental processes (Stumm and Morgan 1996; Zachara and Fredrickson 2004; Cornell and Schwertmann 2003).

“There is plenty of room at the bottom” – this was title of Richard Feynman’s famous talk to the American Physical Society more than half a century ago. The Nobel Laureate, in his historic lecture, discussed the possibility of the direct manipulation of materials on the atomic and molecular level to unleash novel functions. Now, after decades of research, nanoscience faces a historic moment: moving from fundamental research towards a publically available technology, a turning point towards commercialization.

This chapter describes some of the most common methods used to characterize the cell surface properties of the bacterial cells. As a case study, the focus of this chapter is on Sphingomonas spp., Sph2, which is a Gram negative and hydrophilic bacterial strain. The species used in this research was isolated from groundwater at a phenol-contaminated site. This hydrocarbon-degrading strain that can participate in bioremediation of polluted environments belongs to Sphingomonadaceae family. This group of bacteria is unique among Gram-negative cells because of having glycosphingolipids (GSL) instead of the lipopolysaccharide (LPS) layer in their cell wall. To characterize this strain, its surface properties were examined using potentiometric titration, modelling surface protonation sites using ProtoFit, zeta potential measurements, and attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy. There is no published detailed study about cell wall characteristics of Sph2 yet, and this research reports such information for the first time. In addition, to investigate effects of the solution ionic strength on Sph2 adhesion behavior on metal oxides, its biofilm formation on hematite, as the model mineral, was evaluated in three different ionic strengths; ≈200 mM, 100 mM, and 20 mM. The ATR-FTIR analysis showed that despite the unique cell wall chemistry of Sph2 among the Gram-negative strains, its surface functional groups are similar to other bacterial species. Hydroxyl, carboxyl, phosphoryl, and amide groups were detected in Sph2 infrared spectra. The potentiometric titration results showed that Sph2 PZC is approximately 4.3. Optimizing the titration data based on ProtoFit non-electrostatic model (NEM) provided compatible results to the infrared spectroscopy analysis and four pKa values were identified; 3.9 ± 0.3, 5.9 ± 0.2, 8.9 ± 0.0, and 10.2 ± 0.1, which could be assigned to carboxyl, phosphate, amine, and hydroxyl groups, respectively. Zeta potential measurements demonstrated that changing the ionic strength from ≈200 mM to ≈20 mM shifts the zeta potential by ≈−20 mV. Direct observation showed that this alteration in the ionic strength coincides with a tenfold increase in the number of Sph2 attached cells to the hematite surface. This could be attributed to both electrostatic interactions between the cell and surface, and conformational changes of Sph2 surface biopolymers. In addition to reporting Sph2 cell wall characterization results for the first time, this study highlights importance of ionic strength in the cell adhesion to the mineral surfaces, which directly influence biofilm formation, bioremediation, and bacterial transport in aqueous systems.