Characterizing the Cell Surface Properties of Hydrocarbon-Degrading Bacterial Strains, a Case Study
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AbstractThis 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.
DescriptionFrom Handbook of Environmental Materials Management edited by Chaudhery Mustansar Hussain.
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