Galleria mellonella as an alternative in vivo model for investigating Escherichia coli sequence type 131

Abstract

Escherichia coli sequence type 131 has emerged as a globally disseminated cause of community-acquired extra-intestinal infections. Unlike traditional antibiotic resistant strains, members of the ST131 clone derive from the virulent phylogenetic group B2 and encode highly specialised extra-intestinal virulence factors. The objective of this thesis was to test the hypothesis that ST131 are able to combine antibiotic resistance with a higher virulence potential than other E. coli sequence types. To examine the in vivo virulence of ST131, we first established Galleria mellonella as a high throughput model system to assess the in vivo pathogenicity of extra-intestinal E. coli (ExPEC). Findings indicate that viable ExPEC killed G. mellonella larvae in a dose dependent manner and demonstrated a clear correlation between inferred molecular virulence and larval death. Additionally, antibiotic efficacy against ExPEC was examined, with ciprofloxacin prolonging larvae survival in susceptible isolates, further substantiating the suitability of G. mellonella as an in vivo model. When the in vivo virulence of ST131 was examined, larvae mortality rates were highly variable between different clinical isolates. The result of ST131 encoding only moderate virulence-associated profiles indicates that ST131 isolates are neither uniformly virulent nor indubitably more virulent than other ExPEC sequence types. As an alternative explanation behind the recent success of ST131, previous reports have identified the metabolic potential as an important pathogenic factor. When eight E. coli strains underwent extensive metabolic profiling however, no significant difference was observed between metabolic aggregate scores of ST131 and non ST131 isolates. Overall, we have successfully established G. mellonella as an alternative, in vivo model system for investigating ExPEC pathogenicity, while providing further evidence that the epidemiological success of ST131 cannot solely be prescribed to a higher virulence potential. It would appear that additional unidentified factors such as those relating to enhanced colonisation or transmissibility between hosts, may be augmenting ST131 ability to cause serious infections within humans.