Lead scientist Dr Primrose Freestone, from the University of Leicester’s Department of Infection, Immunity and Inflammation, said: “Salad leaves are cut during harvesting and we found that even microlitres of the juices (less than 1/200th 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 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.”
She said the research, published in the journal Applied and Environmental Microbiology, also served as a reminder to consume bagged salad as soon as possible after opening.
“We found that once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge,” Dr Freestone added.
As part of the study, plastic bags were cut into 2cm long sections and tested to see how well salmonella formed clinging “biofilms” on their surfaces. The presence of juice enhanced the bug’s ability to attach to the plastic, researchers said.