Investigating the role of sugar uptake and metabolism in the regulation of Enterohaemorrhagic Escherichia coli virulence

Abstract

Enterohaemorrhagic Escherichia coli (EHEC) is a major foodborne pathogen of the human colon, and cause for zoonotic disease. Transmitted via the faecal-oral route, EHEC exhibits a remarkably low infectious dose, resulting in outbreaks that manifest in bloody diarrhoea, and in extreme cases, acute renal failure. Treatments against EHEC are limited due to conventional antibiotics exacerbating infection. Understanding EHEC pathogenesis is therefore crucial to the development of novel treatment strategies. EHEC have evolved to sense environmental nutrients as “signals” to fine-tune the expression of their primary virulence factor, the Type 3 Secretion System (T3SS), which is essential for host-cell colonisation. These signals include sugars and fats ingested as part of the diet, or by-products of metabolism by the gut microbiota. However, the mechanisms underlying how EHEC exploit these different nutrients are poorly understood. Here, a novel ATP-binding cassette (ABC) transporter in the murine pathogen Citrobacter rodentium, commonly used as a surrogate model for EHEC, was characterised. This system, known to be upregulated during murine infection, is specific for D-ribulose and likely aids colonisation of the mouse gut. Searches in EHEC revealed a similar ABC transporter encoded on a horizontally-acquired genetic element to be significantly enriched amongst EHEC strains. Transcriptionally, the locus was regulated exclusively by L-arabinose, in an AraC- dependent manner. Furthermore, growth on L-arabinose significantly enhanced T3SS expression and the ability to attach to host cells. Deletion of the genes required for L-arabinose uptake, metabolism and associated regulation revealed this phenotype to rely on L-arabinose breakdown and not merely “sensing” its presence in the environment. Collectively, this work suggests L-arabinose metabolism to be important in EHEC pathogenesis through providing a source of nutrition and enhancing virulence gene regulation. It is proposed that these systems and their substrates allow EHEC to outcompete the native gut microbiota, with their downstream metabolism contributing to virulence regulation.