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EHEC prevention: mission ‘safe salad’

Raw fruit and vegetables are popular and healthy, at least as long as they are not contaminated with pathogens such as those that caused the EHEC outbreak in 2011 where sprouted foods were identified as the source of the E. coli outbreak. Researchers from the University of Hohenheim are looking into how the risk of consumers being infected by ready-to-eat vegetables and salads can be minimised.

The EHEC outbreak in summer 2011 was the worst epidemic caused by EHEC in Germany to date and one of the largest outbreaks worldwide. 3850 people were affected and 53 died. The dramatic outbreak was caused by a source that is no stranger, namely an enterohaemorrhagic strain of Escherichia coli, EHEC for short. This is a common pathogenic variant of the E. coli bacteria that are found in the human digestive tract as part of the normal flora. In contrast to the latter, infection with EHEC leads to bloody diarrhoea (entero refers to the intestine and haemorrhagic means bloody). EHEC was described for the first time in 1977 and has since been the cause of several outbreaks. In 1982, it was identified as the cause of a gastrointestinal illness resulting from contaminated beef used in undercooked hamburgers. EHEC contaminations have also been found in spinach. 

Prof. Dr. Herbert Schmidt has been working in the field of medical microbiology for many years and has been specifically focussing on EHEC bacteria for around 20 years: “What was new about the 2011 outbreak was that the disease was highly aggressive. The number of infections that led to severe illness was very high.” Schmidt is the head of the Department of Food Microbiology at the University of Hohenheim and his work specifically focusses on the role of foods in EHEC infections. Schmidt speculates: “It’s possible that the bacteria were so aggressive because they found conditions on the sprouts that made them highly infectious; stress reactions might have triggered them to produce substances that increased their infectiousness.”

What can be done if high temperatures cannot be used to kill the bacteria?

In the laboratory, smears are used to find out whether bacteria are present in food samples. © Universität Hohenheim

As part of an ongoing cooperative project* Schmidt and his colleague from Hohenheim, Prof. Dr. Dr. h.c. Reinhold Carle, are investigating ways to prevent EHEC infections from occurring by way of vegetables and ready-to-eat salads. The project started in November 2012 under the auspices of the Research Association of the German Food Industry (FEI) and is being funded by the AIF (German Federation of Industrial Research Association) on behalf of the German Federal Ministry of Economics and Technology (BMWi) until the end of 2015. As part of the project, Carle, who is in charge of the Plant Foodstuff Technology research section, is developing new technologies with the goal of reducing the transmission of bacteria during the processing of salads and raw vegetables. “Ready-to-eat salads are a particular risk factor because the raw material is chopped into small pieces. Bacteria on the leaves can thus come into contact with the plant sap and stick to the blades of knives from where they can be carried over to produce that is subsequently chopped,” Schmidt explains.

As belt slicers with rotating blades are used for chopping the salads and vegetables, disinfection of the blades after each use is not technically feasible. Therefore, the researchers from Hohenheim are seeking to develop new methods that reduce the risk of contamination with bacteria. The water jet technology seems to be particularly promising. “This technology leads to cleaner cuts; the smoother the cuts are the less sap leaks from the plant and the lower the probability of bacteria being carried over from one vegetable to another. Moreover, the high pressure destroys the bacteria at the cutting edge,” says Schmidt explaining the concept. The researchers are currently assessing the practical application of the water jet technology.

Novel washing and chopping methods with the potential to reduce bacterial contamination

The team of researchers at the University of Hohenheim involved in the project on the prevention of EHEC contamination and infection is specifically focussed on lettuces and other leaf salads. © Universität Hohenheim

In general, “ready-to-eat” means that the salad does not have to be heated, but that it should be washed. Lettuces and other leafy greens are not sterile foods and microorganisms might naturally be present. However, this reveals nothing about whether the bacteria are harmful, harmless or even beneficial. “Around 10,000 CUF (colony forming units) per gramme of leaves is typical, but the number can increase by several powers of ten when the produce is stored for a prolonged period of time,” Schmidt says. Organic salads have much higher bacterial populations, but this alone does not reveal anything about the bacteria’s potential harmfulness either. “Microbial ecologists are also debating whether beneficial bacteria can be found on lettuce and other leafy greens, especially considering the fact that the intake of naturally occurring probiotic bacteria is known to enhance the human immune response. “I think this is an interesting aspect that is worth investigating further,” Schmidt says. 

Nevertheless, the major goal is to prevent EHEC and other pathogenic bacteria contaminations, if possible already before the raw materials are processed. This is why the researchers are also focussing on washing procedures. “Washing lettuces in lukewarm (i.e. 45°C) water for two minutes is more effective in removing bacterial biofilms than washing them in cold water. This does not lead to any loss of quality, provided that the salad is cooled down immediately after washing,” said Schmidt, also highlighting that washing does not remove the entire biofilm. Biofilms consist of microorganisms that are embedded in substances that are excreted by the microorganisms and form a kind of protective envelope.

Need for clarification: How do EHEC bacteria live and survive on vegetables and salads?

Prof. Dr. Herbert Schmidt is the head of the Department of Food Microbiology at the Institute of Food Sciences and Biotechnology at the University of Hohenheim. © Universität Hohenheim

The Hohenheim researchers are pursuing different ideas on how to remove as many germs as possible from salads during processing. “The easiest way would be to use chlorinated water as they do in France. But this is not desirable in Germany where consumers attach great importance to ‘green label’ foods, i.e. simple foods that are processed without the use of additives,” Schmidt says. Moreover, it is not yet clear how well EHEC bacteria can be removed by washing. The FEI project is also focussed on finding an answer to this question. However, the researchers will initially have to find out how EHEC bacteria integrate into biofilms on leaves, and whether and how they manage to survive and reproduce. Do the bacteria form microcolonies? Are they also found in the plants’ stomata? These are two of several questions the Hohenheim researchers hope to be able to answer. 

They will carry out biodiversity analyses before and after washing in order to find out the number of bacteria and species as well as ways to effectively remove them from the leaves as the salads are processed. The so-called vortex method is one of the techniques which the researchers apply to tube systems with laminar flows for the simulation of turbulent flows. Can the resulting shear forces lead to the detachment of the bacteria? The researchers are also investigating the effect of gaseous ozone that is guided over the leaves and whether the washing water can be decontaminated using UV-C radiation before it re-enters the environment. 

Whichever method turns out to be the most effective, the researchers are concerned to find one that does not impair salad quality. This is why they are also analysing the type of plant substances that are released during washing and processing and finding a technique that is best suited to reducing the loss of enzymes and vitamins. And last but not least, the researchers are also focussing on potential changes in texture and colour as washing must of course not result in pale, droopy greens. 

*Cooperative project: "Optimisation of the microbiological quality and physiological properties of ready-to-eat leaf salads and herbs using innovative technologies and microbiological analyses"

Further information:

University of Hohenheim
Prof. Dr. Herbert Schmidt
Garbenstraße 28
70599 Stuttgart
Tel.: +49 (0)711 459-22305
E-mail: herbert.schmidt@uni-hohenheim.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/ehec-prevention-mission-safe-salad