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Title: On the adhesion forces of bacterial surfaces and derivatives
Author: Morales Garcia, Ana Lorena
ISNI:       0000 0004 5349 0891
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2014
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Bacterial adhesion is a multifactorial process, an understanding of which is key in environmental bioremediation and the design of materials for medical applications. The mechanisms that govern cell adhesion must be analysed from the physics point of view in order to obtain quantitative descriptors. The genera Rhodococcus and Pseudomonas are widely spread in nature, forming biofilms. Their adhesion was studied by means of chemical force microscopy (CFM), a technique that relies on the functionalisation of scanning force microscopy (SFM) tips. A pre-requisite of SFM is to have the cells firmly anchored to the surface in order to obtain images and measurements in physiological conditions. A range of immobilisation approaches was attempted, concluding that covalent bonding was the ideal method to attach the studied strains. Force-volume maps of two pseudomonads (Gram negative, hydrophilic) and two rhodococci (Gram positive, hydrophobic) were acquired using four different cantilever functionalities (Si3N4, Au, hydrophobic and hydrophilic self-assembled monolayers) and the results were analysed statistically and fitted using a number of Gaussian curves. The comparisons revealed that the data were highly heterogeneous and that there were small differences between the adhesion forces generated by different combinations of cantilevers and cells. Force-volume maps revealed that, on a given cell, the majority of the bacterial membrane interacted with small adhesion forces with the cantilevers and sparse nanodomains interacted with larger forces. This heterogeneity was explained in terms of biological and chemical differences on the bacterial membrane. Extra-cellular polymeric substances (EPS) form a matrix that encases bacterial cells and promotes their adhesion to surfaces, protecting them from environmental threats. EPS in Pseudomonas contain extra-cellular DNA, but its role in adhesion remains poorly understood. SFM cantilevers were functionalised with DNA strands and probed against Si surfaces under a variety of solutions (H2O; Na+ 2, 20 mM; Ca2+ 1, 10 mM). Specific interactions between the DNA molecule and the surface were detected in a number of force curves, and were usually seen with a higher frequency in the sodium solutions, but the adhesion forces were greater under the calcium solutions. Both observations are in agreement with molecular dynamic simulations that predict a large energy barrier for the attachment and detachment of DNA under calcium (and not so great for sodium) and a bridging effect of the divalent cation between the nucleotide and the surface. It was concluded that eDNA is involved in the biofilm formation process of Pse1, which is mediated by Ca2+.
Supervisor: Mark, Geoghegan ; Stephen, Rolfe Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available