New research on food pathogens is being presented this week at the 21st International ICFMH Symposium ‘Food Micro 2008’ Conference in Aberdeen, Scotland.
Professor Gadi Frankel (Imperial College London) says that even though a small minority of salmonella and E. coli poisoning cases are currently linked to salads, the numbers are increasing. “In their efforts to eat healthy, people are eating more salad products, choosing to buy organic brands, and preferring ‘pre-washed’ bagged salads from supermarkets. All of these factors, together with the globalization of food markets, mean that cases of Salmonella and E. coli poisoning caused by salads are likely to rise in the future. This is why it’s important to get a head start with understanding how contamination occurs now,” he said.
It has long been a mystery as to how these bacteria cling so well to the leaves in these popular bagged salads. Frankel's new study, carried out with Dr Rob Shaw and colleagues at the University of Birmingham, shows how some Salmonella bacteria use the long stringy appendages they normally use to help them ‘swim’ and move about to attach themselves to salad leaves and other vegetables, causing contamination and a health risk. For example, in 2007 a Salmonella outbreak in the UK was traced back to imported basil, and an E. coli outbreak in the USA in 2006 was traced to contaminated pre-packed baby spinach.
Understanding the mechanism that pathogens such as salmonella use to bind themselves to salad leaves is important if scientists are to develop new methods of preventing this kind of contamination and the sickness it causes.
Scientists know that Salmonella and E. coli O157 – a strain of E. coli that can cause serious sickness in humans - can spread to salads and vegetables if they are fertilized with contaminated manure, irrigated with contaminated water, or if they come into contact with fecal matter during the cutting, washing, packing and preparation processes. Professor Frankel's research now shows that some bacteria have developed a new use for their flagella - the long stringy ‘propellers’ they use to move around. The flagella flatten out beneath the bacteria and cling onto salad leaves and vegetables like long, thin fingers.
Unfortunately, our love for bagged salads appears to be setting us on course for new and more challenging microbial illnesses in the future, until science can find a way to remove those sticky "fingers" holding onto the veggie's surface.