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Title: The evolutionary ecology of the PlcR-PapR quorum sensing system in Bacillus thuringiensis
Author: Zhou, Liqin
ISNI:       0000 0004 8497 9147
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2014
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Bacteria can engage in multicellular behaviour via a process called quorum sensing (QS), whereby bacteria monitor population density through the secretion of small, diffusible signal molecules. However, the evolutionary forces that maintain QS have rarely been investigated in naturally co-evolved host-pathogen systems. This thesis investigated the evolutionary ecology of the PlcR-PapR QS system in Bacillus thuringiensis, in which the PlcR regulon controls the production of various extracellular proteins, often involved in virulence, in response to an autoinduced heptapeptide signal PapR. The thesis tested the hypothesis that both signal production and PlcR regulated gene expression are social traits, and measured the invasion of isogenic mutants at varying pathogen doses and mutant frequencies. Productivity in host and infection success was positively correlated with the abundance of wild type in inocula. However, mutants could not outcompete wild type bacteria in vivo. Experiments with homogenized insects indicate that mutants can outcompete wild type bacteria in homogeneous environment. Microscopic observation of insect sections with fluorescent QS cells showed that, in the midgut, bacteria population was founded on isolated patches of 1 to 3 individual cells 24 hours post infection. However, a mixed population consisted of wild type and mutants was evident 48 hours post infection. The results suggested that spatial heterogeneity and population bottleneck imposed by the midgut barriers limited invasion of QS mutants. A polymorphism comprising four distinct communication groups (pherotypes) of the PlcR-PapR QS system was found in the B. cereus group. Strains from the same pherotype exchange information efficiently while strains from different pherotypes communicate far less effectively. Recent theory has suggested that social interactions may account for the within-species diversity of QS systems. Pairwise competition between pherotypes of the studied QS system in vivo revealed fitness to be positively frequency dependent. However, when challenged with a competitive gut bacterium, Enterobacter cloacae, each pherotype exhibited different fitness benefits when competing with other pherotypes in the presence of competitors. The results suggested that environmental dependent fitness has the potential to maintain a stable polymorphism. The results of this thesis confirm that clinical interventions to prevent QS might be beneficial, as diverse virulence factors are often regulated by QS. If QS regulated virulence factors are required for an essential part of infection, inhibiting these behaviours can be helpful.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Quorum sensing ; evolutionary ecology