Masters Degrees (Microbiology)

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Browsing Masters Degrees (Microbiology) by Author "Bester, Elanna"

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    Differential response of sessile and planktonic bacterial populations following exposure to antimicrobial treatment
    (Stellenbosch : Stellenbosch University, 2004-04) Bester, Elanna; Wolfaardt, Gideon M.; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: The ability of biofilms to resist antimicrobial treatment, when planktonic microbes cannot, is of not only fundamental scientific interest, but also a concern in industrial and medical fields. The inability to control biofouling of water distribution networks and products, as well as recurrent infections of implanted medical devices, is not only costly, but also potentially lethal. Several mechanisms whereby biofilms are able to evade antibiotic and biocidal agents have been proposed and investigated, but no universally relevant characteristic has been identified. . Initial investigation, involving BacLightTh ! LIVEIDEAD viability probes, epifluorescence microscopy and image analysis into the ability of natural biofilm and planktonic populations, .cultured in situ in a cooling tower, to survive treatment with a commercial biocide was not conclusive. Subsequent laboratory experimentation with a bacterial isolate from the cooling tower water revealed that the ability of attached biofilms to resist antimicrobial treatment exceeded that of planktonic cells shed from the biofilm. The reduced ability of suspended cells to survive antimicrobial treatment was not statistically significant, compared to that of the biofilm (P = 0.05). This is in contrast to the wealth of literature published on the subject of biofilm antimicrobial resistance The dilution rate in the flowcells in which biofilms were cultivated was more than 100 times higher than the maximum specific growth rate of the test organism. Nevertheless, there was typically more than I x 108 cells/ml in the effluent, suggesting that a metabolically active, rapidly dividing layer of cells existed at the biofilm bulk-liquid interface, from where daughter cells continuously detached. Treatment with an antimicrobial agent resulted in a significant reduction in the viability and number of cells detached from the biofilm, suggesting that this metabolically active layer of the biofilm was more sensitive to antimicrobial treatment, possibly due to a higher specific growth rate. Antimicrobial resistance was shown to be affected by the growth rate for planktonic bacterial populations, with an increased ability to survive, correlated with a decrease in specific growth rate. This supports the contention that growth rate plays a role in the susceptibility of the active layer. The bacterial cells in the layers closest to the attachment surface of the biofilm has frequently been shown to be slow growing, due to nutrient and oxygen limitation, while the outer biofilm layer is more susceptible to unfavourable environmental conditions. It is possible that such differentiation, which results in a responsive outer biofilm layer, provides a mechanism for the protection of the cells in the deeper layers, and thus survival over time. The results presented here support several hypotheses put forth in literature to account for the increased resistance of biofilms towards antimicrobial agents. Future work will include an investigation into changes in the patterns of gene expression when a bacteria becomes attached to a surface, upon subsequent release from the biofilm, and the influence this has on the ability to resist antimicrobial treatment.