Rhodococcus has been used in bioremediation because of its low eco- toxicity, high tolerance to harsh environments, and ability to be cultivated in mixed microbial consortia with certain contaminants as its nutrients. Excretion of extracellular polymeric substances (EPS) allows Rhodococcus to trap and to effectively degrade contaminants. Mycolic acid (MA) which covers the cell wall provides Rhodococcus with a hydrophobic cell surface to contact hydrocarbon contaminant droplets. This work concerns the influence of the conformational change in MA and rhodococcal EPS on their mechanical properties. Neutron reflection revealed that when the solution pH increases, a hydration layer is generated between the bound (hydrophobic) MA (LB _MA) and the silicon substrate, whereas the intermolecular repulsion unfolds the extractable (hydrophilic) MA (LS_MA), and allows water to fill in the formerly folded space. Force spectroscopy using a polystyrene colloidal probe showed that the strength of the adhesion force between a hydrophobic particle and MA is affected by the conformation of MA. The existence of a hydration layer in the MA enhances cell adhesion. Classical DLVO theory indicated that the electrostatic force dominates the long range (a distance larger than the Debye length) interactions between a polystyrene (hydrophobic) particle and MA, whereas the van der Waals force has a negligible influence. EPS generated at the early exponential phase (E EPS) and the late stationary phase (S EPS) of Rhodococcus manifested different physiochemical and mechanical properties. Force spectroscopy using Rhodococcus as a bacterial cell probe suggested that S EPS possess a higher differential capacitance than E EPS do for cells to store charges and energy. The nonspecific binding sites to silicon (an abundant material in the sediments of groundwater) are not evenly distributed; they exist mainly in S EPS close to the cell surface, but rarely in E EPS. Therefore, S EPS have a stronger adhesion to the silicon surface than E EPS do. Contraction and stretch of the EPS chains affect the strength of the adhesion force to a silicon surface. S EPS possess a better resilience against compression than E EPS do, thus retaining water in both S EPS and the inner E EPS. 4
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:566693 |
| Date | January 2012 |
| Creators | Alonso, Edgar Andre Ramirez |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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