No / The motion of an artificial micro-scale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times, it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.

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Call number: LD2668 .T4 1963 C96 / Master of Science

published_or_final_version / abstract / Dentistry / Master / Master of Dental Surgery

published_or_final_version / abstract / Microbiology / Master / Master of Philosophy

A native desert Rhizobium, AZ-M1, was isolated from a nodulated mesquite tree (Prosopis juliflora var. velutina) following inoculation of mesquite seed with a desert soil. This strain and a selected commercial strain (31A5) were used in a greenhouse study to determine their N fixation efficiency against applied N fertilizer. Strain AZ-M1 was a more efficient N fixer than 31A5. The survival rate of the two strains was tested in three different desert soils in a controlled laboratory study. The native strain AZ-M1 grew and survived in the soils over a period of one month. The commercial strain did not grow and the population decreased from 10⁸ cells gm⁻¹ of dry soil to below 10⁴ cells after 14 days. Soil factors affected survival of both strains. The competitiveness of the two strains was compared in a greenhouse experiment. The native isolate out competed strain 31A5 in nodule occupancy regardless of cell number when applied as a mixed inoculant. A high incidence of double occupancy was found in the root nodules when double strain inoculants were applied. The two rhizobia were fast growing rhizobia, lowering the pH of a defined medium. Strain AZ-M1 showed a high intrinsic resistance to 3 antibiotics among 12 tested. Strain AZ-M1 has been shown to be highly effective, fairly competitive and survive better in desert soils than strain 31A5.

No description available.

The organisms in this study were chosen due to their associations with foods and their potential as food borne pathogens. Food borne diseases are an import public health problem in most countries. Bacteria of the genera Campylobacter, Salmonella and Listeria can be transported by poultry and poultry products to humans. Gastroenteritis, typhoid fever, diarrhea, dysentery may originate from the infection. This study was undertaken to determine the incidence of pathogens in a poultry processing plant using polymerase chain reaction and conventional tests and to determine the formation and survival of biofilm cells of food pathogens in trisodium phosphate.

<p>The microbial life of sub-Antarctic islands plays a key role in the islands ecosystem, with microbial activities providing the majority of nutrients available for primary production. Knowledge of microbial diversity is still in its infancy and this is particularly true regarding the diversity of micro-organisms in the Antarctic and sub-Antarctic regions. One particularly important functional group of micro-organisms is the diazotrophs, or nitrogen-fixing bacteria and archaea. This group have not been well studied in the sub-Antarctic region, but play an important role in the nutrient cycling of the island. This thesis explored the diversity of nitrogen-fixing organisms in the soil of different ecological habitats on the sub-Antarctic Marion Island.</p>

The motion of microorganisms presents interesting and diffcult problems ranging from mechanisms of propulsion to collective effects. Experimentally, some of the complicating factors, such as death, reproduction, chemotaxis, etc., can be suppressed through genetic manipulation or environmental control. Nonequilibrium statistical mechanics has been used to study simple models, however proceeding analytically is extremely challenging. Thus simulations, where one has total control over and knowledge of the system, are a compelling method for examining models of their behaviour. In this work I present simulations of minimal, self-propelled particles, while ensuring realistic hydrodynamic behaviour using the lattice Boltzmann method (LBM), a well-studied method for simulating fluid flows that scales linearly in computational effort with the system volume. The derivation of the LBM is reviewed, including the addition of forces in a consistent, accurate manner as well as thermal fluctuations that satisfy the fluctuation-dissipation theorem. It is extended to include singular forces via a regularization of the Dirac δ-function. This is implemented and extensively tested for agreement with low Reynolds number hydrodynamics. The regularized singularities are used to develop an effcient algorithm for pointlike particles which move under the influence of an external force, such as gravity, or thermal fluctuations of the fluid. The method is compared to theoretical results and simulations using a well-studied algorithm that resolves the particle, finding good agreement in the dilute limit and significantly reduced computational requirements. Using the singular forces, we then construct a minimal model for self-propelled particles, that may also experience forces or undergo random changes of orientation (modelling the “run-and-tumble” dynamics observed in swimming bacteria such as E. coli). The collective behaviour of these model swimmers is studied in three situations: sedimentation under gravity; in a central, harmonic trap; and in a Poiseuille flow between parallel plates. For sedimentation, the behaviour is not very different from that expected of non-interacting run-and-tumble particles, except that total collapse to the container bottomwhen the weight of the particles equals the propelling force is prevented by the velocity fluctuations caused by the particles’ activity. The trapped particles, for runlengths comparable to the trap size, self-assemble into a pump-like structure, while for short run-lengths an approximately Gaussian distribution seenwithout hydrodynamic interactions, is maintained. In Poiseuille flows we find the particles orient upstream; forweak flows this results in a net upstreamcurrent. We find significant hydrodynamic effects, in the dilute limit, only when there is some mechanism that causes alignment of the particles.