Secrets of successful pathogen revealed

Two groups of scientists have uncovered key secrets of success of a major pathogen responsible for recent food poisoning outbreaks. The ability of Salmonella bacteria to act quickly, both on an evolutionary timescale and during the early minutes of infection, has been investigated in detail for the first time.

This month more than 1,700 cases of Salmonella food poisoning from chicken were reported in Spain and earlier outbreaks in Europe have been linked to lettuce and eggs.

“For bacteria to do well, they have to react very fast, and we have shown Salmonella to be remarkably dynamic”, says Professor Jay Hinton of the UK’s Institute of Food Research (IFR).

In a study published tomorrow by IFR and Sweden’s Uppsala University, scientists found that Salmonella can evolve at a surprisingly rapid rate by jettisoning superfluous DNA.

One hundred million years ago Salmonella evolved from E. coli bacteria that lived freely in the environment. Salmonella developed the ability to parasitize animals by losing many genes and gaining new ones from other bacteria.

Using DNA microarrays to analyse the results of “experimental evolution”, the scientists tracked Salmonella in real time over 6,750 generations to make the first estimation of the rate of gene loss for any bacterium.

Project leader Professor Dan Andersson says: “Nearly one quarter of the bacteria’s genes could be lost in only 50,000 years. This was a surprise to us as it had been thought this process would take many millions of years”.

green Salmonella bacteria infecting human cells of the gut wall

Some bacteria have evolved the ability to live inside human cells. This picture shows some green Salmonella bacteria infecting human cells of the gut wall (with red membrane and blue nucleus) Image: Dr Isabelle Hautefort (IFR)

In separate research, Professor Hinton of IFR and Professor John Ladbury of UCL (University College London) investigated the response of Salmonella to body temperature. This had not been studied before.

“Bacteria are efficient organisms”, says Professor Hinton. “We found that at low temperatures Salmonella switches off genes required for infection and switches them on once inside a warm animal body. It does not want to expend energy needlessly when waiting to be eaten on a lettuce leaf”.

The team discovered the thermal switch, a protein called H-NS, and found that it allows 532 genes to be activated within minutes. These genes code for functions essential for infection such as the ability to swim and to infect gut cells.

Professor Ladbury believes that as the temperature rises, the protein structure which compacts Salmonella DNA changes shape, allowing gene expression to start.

“These findings help to explain the success of this pathogen in infecting so many different species of animals and reptiles, as well as man”, says Professor Hinton.

Salmonella kills about 1 million people worldwide every year, and now kills more people in the West than any other foodborne pathogen.

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Notes to Editors

Please contact Zoe Dunford for more information, images and an interview with Professor Jay Hinton: 01603 255111 / 07768 164185 zoe.dunford@bbsrc.ac.uk

1) Background

The mission of the Institute of Food Research (www.ifr.ac.uk) is to carry out independent basic, and strategic research on food safety, quality, nutrition and health. It is a company limited by guarantee, with charitable status, grant aided by the Biotechnology and Biological Sciences Research Council (BBSRC).
Uppsala University (www.uu.se) is a leading research university where education grows out of research. Research is pursued in three Disciplinary Domains, across nine faculties: theology, law, arts, social sciences, languages, educational sciences, medicine, pharmacy, and science and technology.
Founded in 1826, UCL (www.ucl.ac.uk) was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. In the government’s most recent Research Assessment Exercise, 59 UCL departments achieved top ratings of 5* and 5, indicating research quality of international excellence.

2) Funding came from

The evolution research was funded by grants from the Swedish Research Council, Uppsala University, the EU Marie Curie Training Site Programme and through the Biotechnology and Biological Sciences Research Council (BBSRC) competitive strategic grant to IFR.
The thermoregulation research was funded by a BBSRC studentship, the BBSRC Core Strategic Grant to IFR and a Wellcome Trust Vacation Scholarship. Prof John Ladbury is a Wellcome Trust Senior Research Fellow.

3) Details of the papers

Bacterial genome size reduction by experimental evolution, PNAS August 23 2005.
H-NS is a part of a thermally controlled mechanism for bacterial gene regulation, Biochemical Journal, 1 October 2005.

4) Salmonella facts

Salmonella food poisoning costs the British economy about £1 billion a year and the US economy almost US$4 billion.

Since the beginning of the 1990s, strains of S. typhimurium resistant to a range of antibiotics have emerged and are threatening to become a serious public health problem.

Since 1885, a total of 2213 strains of salmonella have been identified. They vary in the severity of illness they cause.

Symptoms of salmonellosis (food poisoning caused by Salmonella) are fever, headache, abdominal pain, diarrhoea, nausea and vomiting. In some cases, particularly in the young and very elderly, dehydration can become severe and life threatening.

Salmonella typhimurium can be found in a broad range of animals, birds and reptiles as well as the environment. It causes food poisoning in humans mainly through the consumption of raw or undercooked contaminated food of animal origin - especially meat, poultry, eggs, salad vegetables and milk.

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