University researchers show juices from damaged salad leaves massively stimulate Salmonella growth and salad leaf colonisation

Investigations by Leicester microbiologists have revealed that just a small amount of damage to salad leaves can massively stimulate the presence of the food poisoning bug Salmonella in ready-prepared salad leaves.

Our scientists have discovered that juices released from damaged leaves also had the effect of enhancing the virulence of the pathogen, potentially increasing its ability to cause infection in the consumer.

The research is led by Dr Primrose Freestone from our Department of Infection, Immunity and Inflammation and PhD student Giannis Koukkidis, who has been funded by a Biotechnology and Biological Sciences Research Council (BBSRC) i-case Studentship.

Salad leaf microbe prints on Luria Agar Media plates. Credit: Carl Vivian – University of Leicester

Their research investigates novel methods of preventing food poisoning pathogens from attaching to the surface of salad leaves to help producers improve food safety for consumers. This latest study, published today (18 November) in Applied and Environmental Microbiology, found that juices from damaged leaves in bagged spinach and mixed salad increased Salmonella pathogen growth 2400-fold over a control group and also enhanced their adherence to surfaces and overall virulence, or capacity to cause disease.

Giannis Koukkidis and Dr Primrose Freestone. Credit: Carl Vivian – University of Leicester

Dr Freestone said: “Salad leaves are cut during harvesting and we found that even microliters of the juices (less than 1/200^th of a teaspoon) which leach from the cut-ends of the leaves enabled Salmonella to grow in water, even when it was refrigerated. These juices also helped the Salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.

"This strongly emphasises the need for salad leaf growers to maintain high food safety standards as even a few Salmonella cells in a salad bag at the time of purchase could be become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.

"It also serves as a reminder to consume a bagged salad as soon as possible after it is opened. We found that once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge.

“This research did not look for evidence of salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged."

Leafy green and other salad vegetables are an important part of a healthy diet, providing vitamins, minerals, and dietary fibre. Ready to eat prepared salads are particularly popular, are widely consumed and so of significant economic importance. Over recent years there has however been a number of outbreaks associated with fresh salad produce contaminated with Salmonella and E. coli both in the USA and Europe.  This has triggered considerable interest in effective strategies for controls and interventions measures both in UK industry, the EU and key research funding bodies (http://www.food.gov.uk).

Watch a video interview with Dr Primrose Freestone and Giannis Koukkidis:

Read a Q & A for the general public by the researchers on this study.

• Press release
• The Conversation article 'Salmonella thrives in salad bags'
• ‘Salad Leaf Juices Enhance Salmonella Growth, Colonization of Fresh Produce, and Virulence’ is published in Applied and Environmental Microbiology