Browsing by Author "Hunt, Stephen Michael"

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A 3D model of antimicrobial action on biofilms
(2005) Hunt, Stephen Michael; Hamilton, Martin A.; Stewart, Philip S.
A three-dimensional cellular automata model of biofilm dynamics was adapted to simulate the protection from killing by antimicrobial agents afforded to microorganisms in the biofilm state. The model incorporated diffusion and simultaneous utilization of a single substrate, growth and displacement of cells, detachment, and killing by an antimicrobial agent. The rate of killing was assumed to be directly proportional to the local concentration of substrate available to the microorganisms. Some of the features predicted by this model included development of dynamic, heterogeneous biofilm structures, gradients in substrate concentration leading to regions of substrate depletion in the interior of large cell clusters, variable killing by an antimicrobial agent from one simulation to the next, greater killing of cells at the periphery of cell clusters compared to those cells which were more deeply embedded, and reduced overall antimicrobial susceptibility of cells in the biofilm. These simulations show that substrate limitation can contribute to the protection from antimicrobial agents in biofilms but cannot explain the long-term persistence of biofilm viability that is often observed in practice.
A computer investigation of chemically mediated detachment in bacterial biofilms
(2003-05) Hunt, Stephen Michael; Hamilton, Martin A.; Sears, Joe; Harkin, Gary; Reno, Jason
A three-dimensional computer model was used to evaluate the effect of chemically mediated detachment on biofilm development in a negligible-shear environment. The model, BacLAB, combines conventional diffusion-reaction equations for chemicals with a cellular automata algorithm to simulate bacterial growth, movement and detachment. BacLAB simulates the life cycle of a bacterial biofilm from its initial colonization of a surface to the development of a mature biofilm with cell areal densities comparable to those in the laboratory. A base model founded on well established transport equations that are easily adaptable to investigate conjectures at the biological level has been created. In this study, the conjecture of a detachment mechanism involving a bacterially produced chemical detachment factor in which high local concentrations of this detachment factor cause the bacteria to detach from the biofilm was examined. The results show that the often observed ‘mushroom’-shaped structure can occur if detachment events create voids so that the remaining attached cells look like mushrooms.
Depth profiling using XPS
(Montana State University - Bozeman, College of Engineering, 2000) Hunt, Stephen Michael
Hypothesis for the role of nutrient starvation in biofilm detachment
(2004-12) Hunt, Stephen Michael; Werner, Erin M.; Huang, Baochuan; Hamilton, Martin A.; Stewart, Philip S.
A combination of experimental and theoretical approaches was used to investigate the role of nutrient starvation as a potential trigger for biofilm detachment. Experimental observations of detachment in a variety of biofilm systems were made with pure cultures of Pseudomonas aeruginosa. These observations indicated that biofilms grown under continuous-flow conditions detached after flow was stopped, that hollow cell clusters were sometimes observed in biofilms grown in flow cells, and that lysed cells were apparent in the internal strata of colony biofilms. When biofilms were nutrient starved under continuous-flow conditions, detachment still occurred, suggesting that starvation and not the accumulation of a metabolic product was responsible for triggering detachment in this particular system. A cellular automata computer model of biofilm dynamics was used to explore the starvation-dependent detachment mechanism. The model predicted biofilm structures and dynamics that were qualitatively similar to those observed experimentally. The predicted features included centrally located voids appearing in sufficiently large cell clusters, gradients in growth rate within these clusters, and the release of most of the biofilm with simulated stopped-flow conditions. The model was also able to predict biofilm sloughing resulting solely from this detachment mechanism. These results support the conjecture that nutrient starvation is an environmental cue for the release of microbes from a biofilm.
Theoretical investigation of biofilm detachment and protection from killing using a bacterium level automata model
(Montana State University - Bozeman, College of Engineering, 2004) Hunt, Stephen Michael; Chairperson, Graduate Committee: Philip S. Stewart.
This dissertation presents a three-dimensional dynamic, stochastic computer model of biofilm development, BacLAB, created to theoretically explore conjectures associated with biofilms. BacLAB simulates the life cycle of a biofilm by mimicking the physical and biological behavior of a system with a simple set of experimentally determined "rules" applied to the smallest possible biofilm unit (the cell). These rules, however, lead to patterns on a larger scale. Much as bacterial cells organize themselves in a biofilm as a response to individual spatial conditions, the resulting model structure is produced in a process of self-organization rather than by some predetermined plan for biofilm development. Detachment of cells from a mature biofilm is an important process determining the accumulation of attached cells and allowing for dissemination of the organism. The mode by which cells detach is, therefore, a critical stage in the life cycle of biofilms. Initial simulation studies with BacLAB were used to investigate conjectures associated with detachment resulting from either the accumulation of a metabolic product or the depletion of a metabolic substrate. Results demonstrated that the typical simulated biofilm eventually attains a steady state where biofilm growth was counterbalanced by detachment with cell areal densities comparable to those in laboratory biofilms. Some of the phenomena predicted by BacLAB include sloughing, hollow cell clusters and gradients in solute concentration and growth rate. BacLAB was also adapted to simulate the protection from killing by antimicrobial agents afforded to microorganisms in the biofilm state. It is believed that the reduced susceptibility of bacteria in biofilms is an important factor in the persistence of some chronic infections and the mechanisms of protection are only moderately understood. Because antimicrobials are thought be more effective in killing actively growing bacteria, the rate of killing was assumed to be proportional to the local concentration of the substrate. The results suggest that substrate limitation has the potential to contribute to the reduced antimicrobial susceptibility found in biofilms, but is not adequate by itself in explaining the log-term persistence of biofilm viability observed experimentally.
A three-dimensional computer model of four hypothetical mechanisms protecting biofilms from antimicrobials
(2006-03) Chambless, Jason Daniel; Hunt, Stephen Michael; Stewart, Philip S.
Four hypothetical mechanisms for protection of biofilms against antimicrobials were incorporated into a three-dimensional model of biofilm growth and development. The model integrated processes of substrate utilization, diffusion, growth, cell migration, death, and detachment in a cellular automaton framework. Compared to simulations of unprotected biofilms, each of the protective mechanisms provided some tolerance to antimicrobial action. When the mechanisms were compared to each other, the behaviors of the four protective mechanisms produced distinct shapes of killing curves, non-uniform spatial patterns of survival and cell type distribution, and anticipated susceptibility patterns for dispersed biofilm cells. The differences between the protective mechanisms predicted in these simulations could guide the design of experiments to discriminate antimicrobial tolerance mechanisms in biofilms. Each of the mechanisms could be a plausible avenue of biofilm protection.