Impact of quorum sensing on cell aggregation in enteropathogenic and enterohaemorrhagic Escherichia coli : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosphy in Microbiology and Genetics at Massey University, Turitea campus, New Zealand
sensing has been shown to regulate phenotypic traits such as motility and biofilm production in pathogenic bacteria. Escherichia coli utilise both AI-2 and N-acylhomoserine lactones (AHL) quorum sensing systems to initiate phenotypic switches, such as changes in dissemination, in response to overall microbial population density. In addition HosA, a transcriptional regulator present in pathogenic strains of E. coli, was shown to be important in bacterial dissemination during this study. Deletion of hosA from enteropathogenic E. coli strain E2348/69 resulted in a non-motile population at lower temperatures, an effect that was reversed in the presence of exogenous AHL. Furthermore, addition of the same AHLs to wild-type E2348/69 decreased population motility. Bacterial aggregation has been linked to the motility of the population. Deletion of hosA was shown to increase aggregation, corresponding to an observed decrease in motility. Furthermore, addition of AHL was shown to decrease the propensity of the HosA mutant population to aggregate. Opposing effects were observed in the non-aggregating wildtype population. A hypothetical hierarchical association between HosA and quorum sensing was modelled to explain the relationship between motility and aggregation in E2348/69. It was observed the EAF plasmid is not essential for the formation of A/E lesions on human cell-line HT-29 by E2348/69. Infection of Galleria mellonella revealed increased virulence in highly aggregative populations and further highlighted the effect of HosA and environmental conditions on the pathogenicity of E2348/69. Ruminant animals, in particular cattle, are the main reservoir of enterohaemorrhagic E. coli O157:H7 with infection remaining asymptomatic. By comparison infection in humans can result in a range of sequalae from mild to life-threatening. E. coli O157:H7 is derived from an EPEC progenitor and as expected, dissemination was affected by specific AHLs in a similar manner to E2348/69. However, no aggregation was observed in the presence or absence of signal. This suggested a difference in the genes affected by quorum sensing between enteropathogenic and enterohaemorrhagic E. coli. Development of biocontrol strategies targeting AHL-dependent quorum sensing regulated processes, such as aggregation to reduce E. coli O157:H7 contamination of meat products is possible, based on the data presented in this thesis.