The Time Is Ripe For Salmonella

The ripeness of fruit could determine how food-poisoning bacteria grow on them, according to scientists presenting their work at the Society for General Microbiology's Spring Conference in Dublin. Their work could lead to new strategies to improve food safety, bringing many health and economic benefits.

A wide range of fresh produce has been linked to outbreaks of Escherichia coli and Salmonella enterica including melons, jalapeño and serrano peppers, basil, lettuce, horseradish sprouts and tomatoes. Researchers at Imperial College London are looking at how these bacterial pathogens latch onto fruits and vegetables and establish themselves in the first place.

They have discovered that strains of Salmonella behave differently when attached to ripe and unripe tomatoes. "Bacteria that attach to ripe tomatoes produce an extensive network of filaments, which is not seen when they attach to the surface of unripe tomatoes. This could affect how they are maintained on the surface," explained Professor Gad Frankel who is leading the research. "We are not completely sure yet why this happens; it might be due to the surface properties of the tomatoes or alternatively the expression of ripening hormones."

This is just one example of the subtle interplay between food-poisoning microbes and the fresh produce they contaminate, that determines how pathogens become established in the food chain. "Apart from Salmonella, strains of E. coli are also particularly devious in the way they interact with plant surfaces. They have hair-like appendages and flagella they can use as hooks to successfully secure themselves onto things like salad leaves."

Although fresh fruits and vegetables are recognized as important vehicles that transmit harmful bacteria, they are still important components of a healthy and balanced diet. "By and large, raw fruits and vegetables are safe to eat and provide numerous health benefits. By working out the reasons behind sporadic outbreaks of infections, we can control these better and help maintain consumer confidence. By improving food safety we would also see important economical and health benefits."

Understanding how bacteria interact with fresh produce is a crucial but only the first step, explained Professor Frankel. "Translating research into new policies or methods for decontamination is the challenge for future studies," he said.

Method To Detect Stealthy, 'Hypervirulent' Salmonella Strains

A recent discovery of "hypervirulent" Salmonella bacteria has given UC Santa Barbara researchers Michael Mahan and Douglas Heithoff a means to potentially prevent food poisoning outbreaks from these particularly powerful strains. Their findings, in a paper titled "Intraspecies Variation in the Emergence of Hyperinfectious Bacterial Strains in Nature," have been published in the journal PLoS Pathogens.

Salmonella is the most common cause of infection, hospitalization, and death due to foodborne illness in the U.S. This burden may continue to worsen due to the emergence of new strains that would tax current health-control efforts. To address this problem, researchers sought out - and found - hypervirulent strains that present a potential risk to food safety and the livestock industry.

An international team of scientists - which also included Robert Sinsheimer and William Shimp from UCSB; Yi Xie and Bart Weimer from UC Davis; and John House from University of Sydney, Australia - conducted a global search for hypervirulent Salmonella strains. They were found among isolates derived from livestock, and rendered current vaccines obsolete.

Bacteria behave like a Trojan Horse, exposing their weapons only after initiating infection. "These strains exhibit this behavior in the extreme -essentially having a '5th gear' they can switch to during infection," said Heithoff, lead author of the paper.

Previous efforts to find hypervirulent strains were unsuccessful since bacteria behave much like their less-virulent cousins after environmental exposure. "The trick was to assess their virulence during infection - before they switch back to a less-virulent state in the lab," said Professor Mahan.

Now that researchers know what to look for, they are developing methods to rapidly detect and discriminate the more harmful strains from their less-virulent cousins. The strategy is aided by a special medium utilized by the researchers that forces the bacteria to reveal their weapons in the laboratory - the first step in the design of therapeutics to combat them.

Humans usually get Salmonella food poisoning from eating contaminated beef, chicken, or eggs. However, animal waste can contaminate fields where fruits, nuts, and vegetables are grown, thus posing a particular health concern for vegetarians. The threat is exacerbated when these foods are not cooked. Salmonella control efforts are expensive - recent estimates place this cost up to $14.6 billion annually in the U.S.

As hypervirulent strains pose a potential risk to human and animal health, mitigation efforts warrant researchers' careful attention. "Now that we have identified the problem - and potential solutions - we just need to get to work," Heithoff said.