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Guest, Kerry Teresa
Languages: English
Types: Doctoral thesis
Subjects: Q1
Oxidising biocides are chemical agents grouped together by a similar mechanism of action. They are used in a range of settings to remove unwanted bacterial contamination. Bacterial resistance or decreased susceptibility to biocides was first observed over 70 years ago. The majority of bacteria exist as surface attached communities known as biofilms. Biofilms have a greatly reduced susceptibility to antimicrobials including biocides compared to their planktonic counterparts. For this reason, it is important to study the effect of antimicrobials on biofilms and planktonic cells.\ud The aim of this project is to understand the mechanisms that allow survival and tolerance of bacteria exposed to oxidising biocides by using Salmonella as a representative enteric human pathogen.\ud A range of methods were used to investigate the response of Salmonella enterica to sodium hypochlorite, hydrogen peroxide and peracetic acid. These included culture based methods such as inactivation kinetics and MICs. Flow cytometry, Hoescht assay and GFP reporters were also used to investigate the bacterial response to biocide in the planktonic state. Confocal microscopy was undertaken to investigate the effect of biocides on the structure of biofilms and determine the distribution of survivors within biofilms.\ud Planktonic Salmonella were found to be more susceptible than intact biofilms, but once the biofilms were dispersed this protective effect was reduced. The protective effect was also seen in Salmonella Typhimurium SL1344 which is a poor biofilm former. Hydrogen peroxide exposure resulted in a change to cellular permeability, however this was not linked to efflux through the acrAB system. Flow cytometry demonstrated differential survival upon exposure to different biocides and identified potentially viable but non-culturable populations. Confocal microscopy demonstrated non-uniform distribution of survival of cells within a biofilm. An additional observation is that bacteria responded differently to each of the three biocides tested; there is not one consistent response to biocides with oxidising activity.\ud This research shows the importance of using appropriate methods to test biocides. A key recommendation of this study is that all biocides should be tested against biofilms. This would give a better understanding of the efficacy of the biocides in more realistic and challenging conditions.
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