Molecular dissection of the genetic basis ofEscherichia coli O157 colonisation of the plan tEnvironment

Abstract:

Escherichia coli O157 is a major human pathogen causing bloody diarrhoea, renal failure and death. Its natural niche is the bovine hindgut where it is a harmless commensal. Upon defecation, O157 survives in the environment for several months, allowing the pathogen to contaminate food crops and to re-colonise the herd. We hypothesise that plants play a key role in environmental survival thereby facilitating introduction of O157 into the food chain. Our previous Lawes project enabled identification of several in planta expressed O157 genes that potentially act as plant colonisation factors; it also revealed that O157 can genetically adapt to enhance plant colonisation. We now wish to determine the role of the in planta-expressed genes, establish which of the mutations identified in the adapted strains are responsible for enhanced survival on plant, and detect any changes in soil survival. This work will provide insight into plant colonisation and assist in the development of approaches for limiting food contamination by O157.

Background:

Enterohemorrhagic Escherichia coli (EHEC) O157 is a serious human pathogen that has gained major press coverage over recent years because of its ability to colonise plants, be ingested by humans and causes bloody diarrhoea, renal failure and deaths. The pathogen emerged in 1982 in the USA and was discovered to be present in irrigation water and cow slurries used as plant fertilisers – this is a particular problem when the plant crop treated is a vegetable that is eaten raw eg lettuce, alfalfa shoots. It is a harmless commensal in the rectum of cows and is sloughed off at high cell densities (>10 8 cfu g -1 faeces) and deposited into the environment during defecation potentially promoting spread to other animals via grazing and drinking. The bacteria are known to survive within the natural environment, including pasture, for over a year, which suggests that the pathogen has adapted mechanisms for survival; indeed, the pathogen has been found associated with a number of plants. Research to date has focussed on examining the ecology of the organism and identifying the mechanism of attachment to plants, which suggests that there is a considerable underestimation of the pathogen-plant relationship. The long term bacterial survival of the pathogen in the plant environment strongly argues that the pathogen-plant interaction is more complex than simply attachment and “survival”. Our recent work by a finishing Lawes trust student has used IVET to show that E. coli O157 can survive on pea leaf surfaces where it specifically induces a subset of genes during the interaction. Moreover, experimental adaption of the pathogen on plants showed that new genotypes of the pathogen can evolve with improved plant growth/survival. Genome sequencing has identified a number of mutations in the plant- adapted strains including alterations in sorbose (a fructose isomer) utilisation. The aim of this project is to build upon the findings above in order to understand the importance of the mutated regions in colonisation and pathogenicity. We will determine whether the IVET-identified genes and the genetic changes that occur upon plant-adaption result in altered pathogenicity towards animals, and/or survival in planta or in soil. Thus, our proposed project will reveal whether plant- adapted pathogens pose an increased threat to humans due to enhanced persistence in the plant and soil environment, or because of altered pathogenicity.

Supervisors:

Penny Hirsch / Martin Woodward

Location/s:

Rothamsted Research / University of Reading