An Investigation of Bacterial Interaction Forces and Bacterial Adhesion to Porous Media

Open Access
- Author:
- Camesano, Terri Anne
- Graduate Program:
- Environmental Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 11, 1999
- Committee Members:
- Bruce Ernest Logan, Committee Chair/Co-Chair
Brian Dempsey, Committee Member
Richard Frederick Unz Sr., Committee Member
Susan Louise Brantley, Committee Member - Keywords:
- AFM
atomic force microscopy
bacterial interaction forces
bacterial adhesion - Abstract:
- Bacterial adhesion in porous media was studied through a progression of experiments, from a macroscopic investigation of bacterial transport to a microscopic investigation of bacterial interaction forces. In the macroscopic portion of this work, our goal was to identify conditions under which the transport of bacteria in porous media was the greatest. The effects of flow velocity, cell concentration, cell motility, and solution ionic strength were each examined. The motile bacterium Pseudomonas fluorescens P17 was found to transport furthest at very low fluid velocities, and this transport was greater than could be obtained using non-motile control cells. Cell concentration affected the transport of Pseudomonas fluorescens P17, Burkholderia cepacia G4, and Pseudomonas putida KT2442 in different ways. G4 and KT2442, as well as carboxylated latex microspheres, all exhibit blocking, in which attached particles prevent the further deposition of particles due to repulsive forces. P17 exhibited filter ripening, in which multi-layer films formed since particle-particle interactions appeared to be favorable. The addition of certain chemicals had been suggested as a means of enhancing bacterial transport in porous media. Atomic force microscopy (AFM) was used to image bacterial cells that had been exposed to adhesion-modifying chemicals, to determine the feasibility of using these treatments. Tapping-mode images in air of Pseudomonas stutzeri KC and Burkholderia cepacia G4 revealed that the surfactant Tween 20 and low ionic strength water did not damage cellular morphology, while disodium tetraborate and sodium pyrophosphate caused substantial damage to the cells, and therefore should not be used in bacterial transport studies. Although enhancements in bacterial transport could be made through modification of macroscopic parameters, the conclusions made from these experiments were not sufficient to allow us to formulate a mechanism describing bacterial attachment to soil. It was then concluded that a microscopic approach should be taken to directly measure the bacterial interaction forces that control the adhesion process. AFM was used to measure the interaction forces between individual, negatively-charged bacteria and silicon nitride to determine the effects of pH, ionic strength, and the presence of bacterial surface polymers on interaction forces. Bacterial surface polymers dominated interactions between bacteria and AFM silicon nitride tips. The measured forces were represented well by an electrosteric repulsion model accounting for repulsion between the tip and bacterial polymers, but were much larger in magnitude and extended over longer distances (100's of nanometers) than predicted by DLVO theory. The equilibrium length (Lo) of the polymers was allowed to vary with solution chemistry to account for intramolecular electrostatic interactions between individual polymer units. For Pseudomonas putida KT2442, Lo increased from 229 - 752 nm as pH increased from 4.75 to 8.67, and for G4, Lo increased from 348 nm at pH 2.2 to 1042 nm at pH 7.0. Ionic strength did not affect the equilibrium length of the polymers nearly as much as pH. Partially removing polysaccharides from the bacterial surfaces resulted in lower repulsive forces that decayed much more rapidly. The magnitude of the measured forces in these experiments and the equilibrium lengths predicted by the electrosteric model are comparable to other force measurements and size estimates on polymers and polysaccharides. The results of the AFM force measurements were discussed in terms of their implications regarding bacterial transport in porous media.