# Bacterial virus drives the emergence of deadly bacterial strains

Recent work by Roslin Institute scientists has shown that specific bacterial viruses modify the 'behaviour' of Enterohaemorrhagic E. coli (EHEC) by taking control of an injection system that most EHEC strains need to colonise the gastrointestinal tract.

EHEC strains produce Shiga toxin (Stx) that can be lethal to humans as exemplified by the 2011 outbreak in Northern Germany that resulted in the deaths of 50 people and kidney damage in over 800.

The Shiga toxin is produced by specific bacterial viruses (bacteriophages or 'phages') that infect E. coli and integrate their DNA into the bacterial chromosome where it is replicated along with the bacteria.

Cattle and other ruminants are the usual reservoir hosts for EHEC strains that go on to infect humans. We propose that this altered regulation plus the activity of the toxin may allow the bacteria to persist longer in the ruminant host.

Professor Gally's research showed that the integrating phages introduce a set of genes into the bacterium that help cattle colonisation but which then have serious consequences for humans as an incidental bystander host.

The research results also indicated that the EHEC strain type more likely to cause serious disease in the UK generally contains two of these toxin-encoding phages rather than one. As such, bacterial strain evolution through multiple phage acquisition in cattle may be selecting for bacteria that are more virulent in humans.

Our work highlights the need to understand more about the biology of Shiga toxin-encoding phages and the advantage they provide for bacteria in the ruminant reservoir.

Professor David Gally

The research was a collaboration between groups based at the University of Edinburgh, University of Queensland, Glasgow Biomedical Research Centre, Scottish E. coli VTEC Reference Laboratory and the European Union Reference Laboratory for Escherichia coli in Italy.

Xuefang Xu (1), Sean P. McAteer (1), Jai J. Tree (1), Darren J. Shaw (2), Eliza B. K. Wolfson (1), Scott A. Beatson (3), Andrew J. Roe (4), Lesley J. Allison (5), Margo E. Chase-Topping (6), Arvind Mahajan (1), Rosangela Tozzoli (7), Mark E. J. Woolhouse (6), Stefano Morabito (7) and David L. Gally (1)* 2012. Lysogeny with Shiga toxin 2-encoding bacteriophages represses type III secretion in enterohemorrhagic Escherichia coli. PLoS Pathog. 8(5):e1002672. Epub 2012 May 17.

1. Immunity and Infection Division and
2. Veterinary Clinical Sciences, The Roslin Institute and R(D)SVS, The University of Edinburgh, Easter Bush, Midlothian, EH25 9RG, UK
3. School of Chemistry and Molecular Biosciences and Australian Infectious Diseases Research Centre, The University of Queensland, Brisbane, Australia
4. Institute of Infection, Immunity and Inflammation, College of Medical, Veterinary and Life Sciences, Glasgow Biomedical Research Centre, Glasgow, G12 8TA, UK
5. Scottish E. coli VTEC Reference Laboratory, Department of Laboratory Medicine, Royal Infirmary of Edinburgh, Edinburgh, EH16 4SA, UK
6. Centre for Immunity, Infection & Evolution, Ashworth Laboratories, Kings Buildings, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK
7. European Union Reference Laboratory for Escherichia coli, Istituto Superiore di Sanita, Dipartimento di Sanita Pubblica Veterinaria e Sicurezza Alimentare, Rome, Italy

Author Summary

Many significant infectious diseases that impact on human health evolve in animal hosts. Our work focuses on infections caused by strains of enterohemorrhagic Escherichia coli (EHEC) that cause bloody diarrhoea and life threatening kidney and brain damage in humans as an incidental host, while ruminants are a reservoir host. EHEC strains are infected with bacteriophages that can integrate their genetic material into the bacterial chromosome. This includes genes for the production of Shiga toxins (Stx) that are responsible for the severe pathology in humans. It has been demonstrated that certain EHEC strains are more likely to be associated with human disease and 'supershedding' animals.

The current study has shown that these EHEC strains are more likely to contain two related Stx bacteriophages, rather than one, and that the intercalating bacteriophages take control of the bacterial type III secretion system that is essential for ruminant colonization. We propose that this regulation favours co-acquisition of other genetic regions that encode type III-secreted proteins and regulators that can overcome this control. This finding helps our understanding of EHEC strain evolution and indicates that selection of more toxic strains may be occurring in the ruminant host with important implications for human health.