Spontaneous mutagenesis in stressed <i>Escherichia coli</i>

Mutation processes occurring in starving or stressed bacteria are different from those that are found in growing bacteria. The mutageitic response of E.coli to a number of different stress conditions, including amino acid starvation and challenge with the antibiotic streptomycin has been examined. In an E.coli trpA23 strain starved for tryptophan, the formation of small in-frame deletions was stimulated by the persistence of oxidatively damaged guanine and unrepaired mismatches in the DNA. Most (75%) of the deletions had direct repeats at each termini, one of which was lost in the deletion process. The deletions restored functionality to the product of the trpA gene enabling the mutants to form slow growing colonies on minimal plates lacking tryptophan. During starvation of an E.coli tyrA14 strain WU3610 for the amino acid tyrosine, a new phenotypic suppressor gene (tas) was isolated that specifically complemented the absence of prephenate dehydrogenase activity. The gene is necessary for the process of starvation-associated mutagenesis in this strain; mutants do not arise during starvation in a ray deletion strain. A high proportion (up to 20%) of ancillary mutations within the rpsL gene of E.coli have previously been found in newly arising streptomycin dependent mutants and have been shown to confer a selective advantage in established strains. This study shows that they also confer an advantage in mutants containing mixed wild type and streptomycin dependent ribosomes in the presence and absence of streptomycin. However, this selective advantage is still not enough to account for the rate at which these ancillary mutations are recovered and we need to assume a mutation rate of 10-3 to 10-4/gene/replication to explain the observed frequency

Modelling the effects of temperature-dependent material properties in shear melt layers

The mechanisms responsible for ignition of explosive materials in response to low energy stimuli, known as \insults" in the literature, are still not well understood. It is in general believed that explosive ignition is of thermal origin, with mechanical energy being converted into heat energy in localised regions, forming so-called \hot spots". When an explosive sample is subject to a mechanical insult pre-existing, or new, microcracks will be in compression and shear. It is possible for such microcracks to grow in size if the local stress is great enough and, due to friction between solid surfaces, heat is released during the growth process. Subsequent to sufficient heat release, the crack surface temperature will be raised to the solid melting point and a thin sheared melt layer will be formed, separating the solid surfaces. This thin melt layer will continue to be heated through viscous dissipation and subsequent chemical reaction, and is thought to be a prime location for so-called hot spot generation. Mechanical insults, resulting from low-speed impacts which shear an explosive, have been identified as a possible ignition source. However, modelling such an ignition mechanism numerically with hydrocodes proves to offer some considerable challenges. To supplement the numerical approach, we develop an analytical model of the shearing, melting and subsequent ignition of an explosive material. We consider the melting of a thin viscous layer of explosive material due to an applied shear in an idealised planar geometry. The model accounts for self-heating due to mechanical dissi- pation, and a single-step Arrhenius reaction is used to describe the heating of the explosive due to subsequent chemical reaction. A solution is sought by considering perturbations from a melt layer of uniform width. In particular, we consider the effects of modelling the temperature dependence of the liquid viscosity and specific heat are studied. In contrast to previous work which does not account for the temperature dependence of material properties, it is shown that allowing the viscosity to vary with temperature can lead to non-uniform mechanical heating in the layer to leading order. Such localised heating may be associated with generation of localised hot spots which give rise to ignition.</p

A Model to account for the effects of Friction during Explosive Pinch

Safety is of paramount importance in the handling, processing and storage of explosives. Mechanical insults resulting from low-speed impact, that crush and pinch an explosive, have been identified as a possible ignition source. However, modelling such an ignition mechanism numerically with hydrocodes proves to offer some considerable challenges. Here we develop a model for the pinching of an explosive cylinder between two flat plates which accounts for the effects of friction at the contact between the plates and the explosive. An ad hoc analytical method of the axial pinching of an explosive cylinder by two flat plates moving at constant speed is developed and discussed in [1]. In this formulation it is assumed that as the material is compressed it is in perfect plastic flow under adiabatic conditions. The explosive reaction is modelled using a simple Arrhenius Law. The heating of the explosive due to mechanical heating and self heating due to the reaction are calculated. In the analysis presented there is no treatment of friction at the contact region between the plate and explosive. As a result of this simplification the dissipation calculated is constant throughout the sample. This is contrast with experiments conducted at AWE in which non-uniform heating is observed [2]. Sherwood and Durban [3] investigated the squeezing of a non-reactive viscoplastic solid in the presence of friction. It is suggested that their paper may form a strong basis to explore frictional effects in the configuration posed in [1]. Here we adopt the approach taken in [3] to describe the mechanical behaviour of an explosive sample subject to axial compression, and then introduce a simple Arrhenius Law, as in [1], to model the reaction. The work presented allows us to investigate the effects of frictional heating during compression and arrive at an improved model of the so called Pinch Test

Shear localisation during deformation of reactive thermo-visco-elastic-plastic materials

A one-dimensional model for the initiation of shear bands in a reactive material is developed, with an Arrhenius source term to model the chemical reaction occurring in the band. An inhomogeneity in the heat flux is used as the stimulus for localised plastic deformation, and the problem is treated as a perturbation to the elastic solution. In the analysis, the thin zone of localisation is identified as a boundary layer. It is found that the behaviour of the perturbations to the temperature and stress in the band are governed by three dimensionless parameters which are known in terms of various material properties. The resulting equations are solved numerically and a criterion for the onset of shear banding is discussed

Mathematical modelling and investigation of explosive pinch, friction and shear problems

The mechanisms which lead to the accidental ignition of explosive materials in response to low energy stimuli are still not well understood. It is widely agreed that localised regions of increased temperature, so-called `hot spots', are responsible. Many mechanisms for hot spot generation have been suggested as a result of experimental studies, but the understanding of such processes remains incomplete. In this thesis, we use asymptotic and numerical techniques to investigate hot spot mechanisms, with a particular focus on those arising from impacts which pinch and shear explosives. First, a model which accounts for the effect of friction as an explosive material is pinched between two at plates is developed. An analytical solution is found by exploiting the small aspect ratio of the explosive sample. Numerical solution of the thermal part of the problem demonstrates that our model is able to predict important features observed in experiments, such as additional heating near to the plates. We then go on to study how the presence of a chemical reaction affects the development of shear bands as explosive materials are deformed. Through a boundary layer analysis, we are able to extract key non-dimensional parameters which control the development of shear bands in explosives, and discuss how this may inform the design of materials that are less susceptible to accidental ignitions due to mechanical insults. Finally, we investigate how molten layers of explosive, which can form between sliding surfaces during shear deformation, may act as a site for hot spot generation. In particular, we consider how the inhomogeneous nature of explosive materials affects the propagation of the melt front. Through a lubrication-type analysis, we demonstrate that the melt front is unstable to perturbations in the presence of a chemical reaction, and that material non-uniformities lead to localised heating within the molten layer

A mechanism for hot spot generation in a reactive two- dimensional sheared viscous layer

A two-dimensional model for the non-uniform melting of a thin sheared viscous layer is developed. An asymptotic solution is presented for both a non-reactive and a reactive material. It is shown that the melt front is linearly stable to small perturbations in the non-reactive case, but becomes linearly unstable upon introduction of an Arrhenius source term to model the chemical reaction. Results demonstrate that non-uniform melting acts as a mechanism to generate hot spots which are found to be sufficient to reduce the time to ignition when compared with the corresponding one-dimensional model of melting

Literacy skills of Australian Indigenous school children with and without otitis media and hearing loss

This study examined the relationship between reading, spelling, and the presence of otitis media (OM) and co-occurring hearing loss (HL) in metropolitan Indigenous Australian children, and compared their reading and spelling outcomes with those of their non-Indigenous peers. OM and HL may hinder language development and phonological awareness skills, but there is little empirical evidence to link OM/HL and literacy in this population. Eighty-six Indigenous and non-Indigenous children attending pre-primary, year one and year two at primary schools in the Perth metropolitan area participated in the study. The ear health of the participants was screened by Telethon Speech and Hearing Centre EarBus in 2011/2012. Participants’ reading and spelling skills were tested with culturally modifi ed sub-tests of the Queensland University Inventory of Literacy. Of the 46 Indigenous children, 18 presented with at least one episode of OM and one episode of HL. Results indicated that Indigenous participants had significantly poorer non-word and real word reading and spelling skills than their non-Indigenous peers. There was no significant difference between the groups of Indigenous participants with OM and HL and those with normal ear health on either measure. This research provides evidence to suggest that Indigenous children have ongoing literacy development difficulties and discusses the possibility of OM as one of many impacting factors