A molecular and biochemical analysis of a novel D-serine sensory system in Escherichia coli O157:H7

Connolly, James P.R. (2015) A molecular and biochemical analysis of a novel D-serine sensory system in Escherichia coli O157:H7. PhD thesis, University of Glasgow.

Abstract

Escherichia coli is a diverse bacterial species found largely as a harmless commensal in the gastrointestinal tract of warm-blooded mammals. However, an array of highly adapted E. coli pathotypes have evolved over time capable of causing a variety of niche specific diseases, both intestinally and extraintestinally. This ability to cause disease results from the adaptation of the core genome to the host and the acquisition of horizontally acquired virulence factors. Furthermore, these foreign virulence factors are integrated into the regulatory network of the cell allowing niche specific competitive advantages for the emerged pathogen. Enterohaemorrhagic E. coli O157:H7 is a dangerous pathogen capable of causing haemorrhagic colitis and the potentially fatal haemolytic uremic syndrome. This pathogen is a harmless coloniser of ruminants whereas humans are extremely susceptible with transmission via the faecal-oral route, commonly associated with contaminated food products.

It is becoming apparent that the carriage of virulence factors is only one element to the adaptive nature of E. coli pathogens, with the specific response to niche specific signals governing when and where these virulence factors are utilised. O157:H7 utilise a type 3 secretion system (T3SS) as their major colonisation factor, which facilitates subversion of and subsequent intimate attachment to the human intestinal epithelium. This T3SS is encoded on a pathogenicity island known as the locus of enterocyte effacement (LEE) that encodes all the necessary components of a T3SS and a number of effector proteins. The LEE is tightly regulated in a temporal manner that is highly responsive to components of the intestinal physiology - namely nutrients, metabolites and hormone-like signals. Interestingly, despite the LEE encoded T3SS being capable of mediating attachment to a diverse variety of cell types in vitro, this colonisation factor is exclusively utilised by intestinal E. coli pathotypes but the mechanistic reasoning behind this is unknown.

In this work, the impact of the host metabolite D-serine on O157:H7 is explored in detail. D-serine is highly abundant in sites commonly colonised by extraintestinal E. coli pathotypes, such as the urinary tract and the brain. Furthermore, it acts as a positive fitness trait and regulator of virulence factors within these niches. Conversely, intestinal strains of E. coli are largely unable to metabolise D-serine but its effects on their gene expression has not previously been investigated. Here, it is demonstrated that D-serine selectively affects gene expression in O157:H7 by repressing the LEE encoded T3SS and activating the SOS stress response. The toxicity of D-serine was entirely dependent on intracellular accumulation of this metabolite however the ability of D-serine to repress the LEE was found to be independent of its ability to be metabolised. Comparative genomic analysis revealed that carriage of both the LEE and the D-serine tolerance locus (dsdCXA) is an extremely rare event attributed to the apparent incompatibility between the two loci. It is proposed that the negative effects of D-serine on the LEE limit pathotypes such as O157:H7 to the gastrointestinal tract by forcing the evolutionary loss of dsdCXA, demonstrating the importance of co-operation between horizontally acquired and core pathogenic elements in defining niche specificity. A novel D-serine sensing system was also identified and characterised in O157:H7. This system includes a D-serine transporter, YhaO, and a LysR-type transcriptional regulator, YhaJ, which are absolutely required for expression of the LEE in O157:H7. This system is highly conserved in all E. coli and further demonstrates the adaptive ability of the core genome to perceive and respond to important environmental signals that define specific niches.

Collectively, this thesis describes the mechanistic basis of D-serine sensing in O157:H7 and the physiological relevance of D-serine sensing for diverse E. coli pathotypes. This work provides a strong framework for further research both by revealing novel insights into bacterial evolution and also creating potential targets for anti-bacterial therapeutics.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Escherichia coli, O157:H7, regulation, virulence, type 3 secretion, D-serine, niche, environmental sensing
• Subjects: Q Science > QR Microbiology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Infection & Immunity > Bacteriology
• Supervisor's Name: Roe, Dr. Andrew J and Cogdell, Professor Richard
• Date of Award: 2015
• Depositing User: Mr James PR Connolly
• Unique ID: glathesis:2015-6313
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 21 May 2015 13:28
• Last Modified: 21 May 2015 13:57
• URI: https://theses.gla.ac.uk/id/eprint/6313