Pre-packed and bagged salads are "breeding grounds" for Salmonella, according to a new study.
Scientists at the University of Leicester measured levels of salmonella in salad bags and analysed the way the bacteria grew on broken leaves as well as how it attached itself to plastic bag surfaces.
The findings, published in the journal Applied and Environmental Microbiology, found that juice from broken leaves increased salmonella growth in water by 110%.
When the juice was added to a medium supporting the growth of salmonella, a 2,400 fold increase in growth was observed.
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. Salad leaves from cos, baby green oak, and red romaine lettuce, as well as spinach and red chard, were analysed for the study.
An initial contamination of 100 Salmonella bacteria would increase to 100,000 within five days - more than an infectious dose.
Lead scientist Dr Primrose Freestone, from the University of Leicester's department of infection, immunity and inflammation, said: "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."
Off the back of the study, researchers advise consumers to eat bagged salad as soon as possible after opening it.
Salmonella causes food poisoning, which results in symptoms such as diarrhoea, stomach cramps, nausea, vomiting and fever. These symptoms usually develop between 12 and 72 hours after becoming infected, and illness usually lasts between four and seven days.
Over 320 cases of salmonella were reported in Ireland in 2013.
In 2011, a salmonella outbreak that affected more than 2,000 people across Europe was traced back to bean sprouts and this year, officials in England traced an outbreak that killed two people back to bags of rocket leaves.