Thesis (M.S.)--University of Delaware, 2007. / Principal faculty advisors: Charles E. Mason and J. Lindsey Flexner, Dept. of Entomology & Wildlife Ecology. Includes bibliographical references.

Thesis (Ph. D.)--University of Chicago. / Reprinted from the Physical review, v. 28, no. 2, Feb. 1909. Includes bibliographical references. Also available on the Internet.

Thesis (S.T.M.)--Dallas Theological Seminary, 1985. / Includes bibliographical references (leaves [56]-60).

Thesis (Ph. D.)--University of Sydney, 2007. / Title from title screen (viewed 15 January 2009). Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Information Technologies, Faculty of Science. Includes bibliographical references. Also available in print form.

No description available.

<i>Pseudomonas aeruginosa</i> displays high levels of tolerance and resistance to many antimicrobial agents. Much of this tolerance is related to the nature of the Gram-negative cell envelope and in particular, the outer membrane. The outer membrane plays an important role in excluding harmful molecules from the cell, whilst being selectively permeable to other solutes via its implanted proteins (outer membrane proteins or OMPs). In order to exert their antibacterial action, antimicrobial agents must enter the cell and attain sufficiently high concentrations at their target site(s). The OMPs are highly sensitive to environmental changes and have a physiological ability to respond to such changes. It is thought that the altered cell envelope structure contributes to the accessibility of antimicrobial agents into the cell interior and resistance to such agents is related to over expression or loss of certain OMPs. Brozel and Cloete (1994) observed a gradual increase in tolerance to increasing concentrations of biocide upon exposure of <i>P. aeruginosa</i> to KathonTM, a commercial biocide containing 1.15% v/v 5-chloro-N-methylisothiazolone (CMIT) and 0.35% v/v N-methylisothiazolone (MIT). This adaptation was associated with the concurrent disappearance of a 35kDa OMP, designated T-OMP. Therefore, they concluded that the biocide entered the sensitive cells via the T-OMP and that the observed resistance was the result of the absence of this OMP. The aim of this investigation was to induce tolerance in cultures of <i>P. aeruginosa</i> PAOl towards the pure active forms of the three isothiazolone biocides 1,2-benzisothiazolone (BIT), MIT, CMIT and the thiol-interactive agent thiomersal (used as a positive control). An increase was observed in the minimum inhibitory concentrations (MIC) of all four biocides by at least 58% between the sensitive and resistant cultures. In some cases the percentage increase in MIC was in excess of 150%. However, when the tolerant cells were removed from the presence of the biocide, the MIC began to decrease, indicating a loss in tolerance. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDSPAGE) analysis of the OMP profiles from the tolerant-induced cultures illustrated the loss of T-OMP in all cases. Analysis of the sensitive and resistant cultures using twodimensional polyacrylamide gel electrophoresis (2D-PAGE) indicated that the T-OMP disappeared in the tolerant cultures. However, these observations also suggested that other outer membrane alterations occur concurrently in T-OMP depleted tolerant cells. Investigations into the cross-resistance of the resistant cultures towards the other test biocides, indicated that resistance was, to some extent, transferable, once it had been developed towards one member of the biocide group. Following routine passaging of the resistant cultures on gradient plates two distinct colonial morphologies were observed, mucoid and non-mucoid. An increase in the cell surface hydrophobicity was noted between the mucoid and non-mucoid cultures, which indicated a loss or reduction in the B-band OPolysaccharide. However, there were no observable differences in the lipopolysaccharide banding patterns between the mucoid and non-mucoid cells. These observations suggested that other alterations were occurring in the tolerant cells upon exposure to biocide, over and above the simple disappearance of T-OMP. Therefore, it is suggested that the observed tolerant development in biocide exposed cells, was not solely due to the loss of T-OMP. Investigations into Gram-negative bacteria isolated from contaminated industrial samples preserved with isothiazolone compounds exhibited higher MICs towards the preservative biocides than would normally be expected in the species of bacteria isolated and identified. However, there were no observable alterations in their OMP profiles.

No description available.

No description available.

M.Sc. / The possible resistance of the red spider mite, Tetranychus cinnabarinus, on cotton to the acaricides triazophos and profenofos was studied in the Groblersdal/Marble Hall area. During the 1980 and 1981 seasons T. cinnabarinus was collected from cotton in four districts where mite infestations had been controlled the preceeding two seasons with up to three triazophos sprays per year. A laboratory culture of T. cinnabarinus served as the control and the response of these mites to different concentrations of acaricides were compared with those of the four field cultures. The toxicity of triazophos and profenofos was evaluated by using the slide-dip technique and mortality readings were taken 24 hours after exposure to the acaricide solutions. The data were subjected to a pro bit analysis. Where pro bit lines could not be fitted second degree polinomial regressions were fitted through the log dose and empirical probits. During 1980 there was little difference between the susceptibility of the four field cultures to triazophos. However, the regression lines of all four cultures when compared with those of the control culture indicated resistance to triazophos. After these field cultures had been reared under controlled conditions and not exposed to pesticides for a year all signs of triazophos resistance disappeared. The four field cultures collected in 1981 showed great variability in their response to triazophos but no indication of resistance was observed. This may be because the recommended cotton pest control programme was more closely followed by the cotton producers in 1981 and fewer acaricide sprays were applied. The accuracy of the mortality readings made 24 hours after exposure to triazophos was investigated by taking readings at intervals of 8 hours. An analysis of the results indicated that the mortality curves calculated from all readings were not significantly different and consequently mortality readings were made every 24 hours after exposure to the acaricide solution. A second laboratory strain of T. cinnabarinus, obtained from the Fruit and Fruit Technology Research Institute in Stellenbosch, and regarded as susceptible to organo phosphates was also tested for resistance to triazophos. The regression lines obtained for this culture was similar to that obtained from the 1980 field cultures. This was taken as an indication of possible resistance and this culture was therefore not used as a con trol in this study. The control culture and the Groblersdal/Marble Hall field cultures did not differ significantly in their susceptibility to profenofos. However, profenofos is not widely used for the control of T. cinnabarinus on cotton and these results indicate that it can still be used effectively for the control of T. cinnabarinus. A third acaricide, amitraz, was also examined to determine its effect on T. cinnabarinus. The slide-dip technique was found to be unsuitable for the determination of mite resistance to amitraz. This acaricide is slow acting and an observation interval longer than 24 hours is required to obtain the necessary mortality data. It is difficult to exclude the effects of starvation when the slide-dip technique is used for prolonged periods and consequently amitraz was not tested further. An alternative method for the effective evaluation of this acaricide must be developed. From this study no definite evidence of resistance of T. cinnabarinus in the Grtoblersdal/Marble Hall cotton growing area to triazophos or profenofos was found. It was concluded that both can still be used effectively in the control programme for mites on cotton.

A conjugation system in Staphylococcus aureus has been investigated and shown to be determined, at least in part, by genes carried on plasmids. Conjugation required cell-to-cell contact but not calcium ions. The frequency of conjugation depended on the recipient used and on the incubation conditions. Two conjugative plasmids were mapped by restriction enzyme analysis but experiments to clone the conjugation-determining region were unsuccessful although separate regions specifying gentamicin resistance, ethidium bromide resistance and cadmium resistance were cloned. The gentamicin resistance determinant was probably part of Tn4001. Deletion of various sized pieces of DNA from one of the plasmids resulted in reduction of its ability to specify conjugation but no specific part of this plasmid could be implicated in the process. Further experiments led to the conclusion that this particular plasmid (p8325-4) is probably not self-transmissible but transferred by a phage-mediated system. Strains of Staphylococcus aureus produced a pheromone-like substance that elicited a clumping response in Streptococcus faecalis but no evidence was found for the involvement of staphylococcal conjugative plasmids in this. The conjugative plasmid, p8325-2, mobilized a small plasmid (pT181) but not a chromosomal gene. Insertion of transposon Tn551 was used to produce mutants of the conjugative plasmid p8325-2. Some twenty-six mutants were studied and the position of Tn551 in them mapped. There were preferred regions of insertion for Tn551 and twenty out of the twenty-six mutants had altered ability to conjugate. One showed a significantly higher frequency of conjugation and the other nineteen, all with substantially lower frequencies of conjugation, were mapped to two well-separated regions of the plasmid. Similarity between the locations of these putative regions and those reported for some other conjugative plasmids from staphylococci is striking and suggests a common origin.