Date of Award

May 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Yan-Yeung Luk

Keywords

Bacterial Biofilms, Maltose derivatives, Non-microbicidal approach, Pseudomonas Aeruginosa, Rhamnolipids

Subject Categories

Physical Sciences and Mathematics

Abstract

Since the serendipitous discovery of the first antibiotic, the "wonder drug" penicillin by Alexander Fleming, bacteria over time have slowly developed resistance to most antibiotics through three well coordinated processes. Firstly, bacteria can evolve their genetic makeup to become resistant against antibiotics; Secondly, bacteria can relay the modified antibiotic resistant genes to other bacteria and other species through a process called conjugation. Thirdly, bacteria quickly give up their individuality to become a part of a team to form surface attached multicellular communities known as biofilms. Bacteria residing within biofilms are protected by a layer of slime which renders the bacteria one thousand fold more resistant to the action of antibiotics. Nearly eighty percent of bacterial infections are associated with biofilms and therefore understandably, biofilms are considered as one of the seven most important health issues facing mankind in the 21st century. The focus of research work presented here is to discover small molecules that can control multiple microbial multicellular behaviors. The fundamental approach was to deploy small molecules that do not kill the bacteria (nonmicrobicidal), but are able to modulate bacterial multicellular behaviors, like biofilm formation and swarming motility, which are not essential for bacterial survival but are critical for infections. Consequently, the use of such nonmicrobicidal agents is less likely to induce evolution of bacterial genes. The use of two different kinds of nonmicrobicidal agents, maltose derivatives and brominated furanones, as modulators of different bacterial multicellular behaviors has been demonstrated. Rhamnolipids secreted by Pseudomonas aeruginosa are biosurfactants that are known to be essential for at least three multicellular behaviors of P. aeruginosa, biofilm formation, biofilm dispersion and swarming motility. Maltose derivatives, which are structurally related to

rhamnolipids were synthesized and found to be nonmicrobicidal to the growth of P. aeruginosa, Escherichia coli and Staphylococcus aureus. Maltose derivatives were effective at inhibiting the initial adhesion, biofilm formation and at dispersing pre-formed biofilm of P. aeruginosa. Maltose derivatives were capable of both modulating the swarming motility of wild type P. aeruginosa (PAO1) and activating swarming of a nonswarming P. aeruginosa mutant, rhlA. Although, the maltose derivatives were able to inhibit the biofilm formation, these agents were not effective at either inhibiting the initial adhesion or dispersing the preformed biofilms of both E. coli and S. aureus. Brominated furanones are known to disrupt bacterial chemical communication process known as quorum sensing (QS). Here the mechanism of action of brominated furanones on both E. coli and P. aeruginosa was explored. The presence of a methyl substituent either on the furanone ring or on the exocylic vinyl bond was identified as an important structural element for maintaining nonmicrobicidal action. The protein SdiA of E. coli was found to be critical for antibiofilm activities of brominated furanones against E. coli. The lasI protein on P. aeruginosa is a known homologue of the E. coli SdiA protein. It was found that the brominated furanones were antagonistic to the las QS system but were agonistic to the rhl QS system of P. aeruginosa. The nonmicrobicidal agents presented here, maltose derivative and brominated furanones offer new approaches for controlling biofilm and bacteria-related problems.

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Open Access

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