New science unlocks secrets of salmonella success
surprisingly rapid rate "by jettisoning superfluous DNA", report UK
Researchers at the UK's Institute of Food Research (IFR) claim to have unlocked the key secrets of success for this pervasive pathogen responsible for a raft of food poisoning outbreaks.
Carried in eggs, poultry, raw milk and chocolate the salmonella bacteria can cause bloody diarrhea, abdominal cramps and in extreme cases kidney failure.
Incidents are a major problem in most countries across the globe, leading to hefty costs for the public and private sector.
Recent estimates from the US put total annual costs (medical care and lost productivity combined) of the pathogen at a massive $2.3 billion (€1.85bn). And salmonella food poisoning costs the British economy a considerable £1 billion a year.
And in industrialised countries, the percentage of people suffering from foodborne diseases each year has been reported to be up by 30 per cent.
"For bacteria to do well, they have to react very fast, and we have shown Salmonella to be remarkably dynamic," comments Professor Jay Hinton of the UK's Institute of Food Research.
One hundred million years ago Salmonella evolved from E. coli bacteria that lived freely in the environment. Salmonella developed the ability to parasitise animals by losing many genes and gaining new ones from other bacteria, explains the IFR.
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.
"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," says project leader Professor Dan Andersson.
In separate research, Professor Hinton of IFR and Professor John Ladbury of UCL (University College London) tread new ground by investigating the response of Salmonella to body temperature.
They found that "at low temperatures Salmonella switches off genes required for infection and switches them on once inside a warm animal body," reports Professor Hinton.
It does not want to expend energy needlessly when waiting to be eaten on a lettuce leaf, adds the professor.
According to IFR, 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.
Last year the UK's Food Standards Agency (FSA) and the Health Protection Agency (HPA) announced plans to clamp down on salmonella. Their investigations revealed that since 2002 the country had experienced more than 80 outbreaks of Salmonella enteritidis, with 2000 confirmed and an estimated 6000 potential cases: many of which were linked to Spanish eggs used in the catering trade.
The UK is still recovering from wide outbreaks of this foodborne pathogen in the 1980s that knocked the local egg industry. Figures now show that the number of cases in England and Wales have nearly halved since this time, dropping from 16,047 cases in 1998 to 9757 cases in 2003; mainly due to industry control programmes, including the vaccination of chicken flocks.
Sweden is the country with the lowest occurrence of salmonella in the world; the food industry's methods have aroused considerable interest from US and European food producers.
The Swedish method attempts to make 'a polluted product clean', the control points are moved backwards in the production chain, including the egg production site, as well as a strong focus on hygiene related matters.
The studies, Bacterial genome size reduction by experimental evolution and H-NS is a part of a thermally controlled mechanism for bacterial gene regulation are published by IFR and Sweden's Uppsala University in the 23 August edition of PNAS and the 1 October 2005 of Biochemical Journal respectively.