Bagged salad leaves “breed Salmonella” – spinach the worst

 

It looks fresh and is so convenient, but bagged salad is a big risk for food poisoning, according to a new study from the University of Leicester.

 

It shows that small amounts of damage to salad leaves in bagged salads encourage the presence of Salmonella enterica. Juices released from damaged leaves also enhance the pathogen’s ability to attach to the salad’s plastic container.

 

Earlier this year Public Health England warned the public to thoroughly wash leaves of pre-packaged salad after 151 people fell ill, with two people dying and 62 needing hospital care, mostly in the south-west of England, after an outbreak suspected to have originated with bags containig rocket leaves.

 

In the study, published in Applied and Environmental Microbiology, salad juices in water – to mimic the grocery salad bag environment – more than doubled motility, or movement of individual Salmonella bacteria, abetting salad leaf colonisation. In the course of a typical five-day refrigeration storage time, around 100 Salmonella bacteria multiplied to approximately 100,000. “That’s more than an infectious dose”, said co-author Dr Primrose Freestone, PhD, associate professor in clinical microbiology.

 

Salad juices also boosted formation of biofilms on salad leaves. Microbial biofilms generally cling tenaciously to the surfaces they coat – medical implants, stainless steel, or teeth, in the form of dental plaque – and Salmonella biofilms on salad leaves are no exception. They are powerfully resistant to being washed off.

 

Yet surprisingly, the normal microbial flora on salad leaves did not respond to leaf juices, suggesting that the leaf juices give Salmonella a marked advantage in colonising salad leaves as compared to competing bacteria, according to the report.

 

Salad leaf crops are usually grown in open fields where they can be exposed to Salmonella via insects, bird droppings and manure, among other sources. While outbreaks of Salmonellosis due to such  contamination are uncommon, they are nonetheless a public health problem. Such outbreaks may occur despite practices used to mitigate the problem, such as irrigation with clean water, good hygiene, leaf washing, and the like, said Prof Freestone. In fact, salad leaves can acquire Salmonella from recycled wash water, she said.

 

Won’t come out in the wash

 

Moreover, earlier studies have shown that Salmonella are so powerfully attracted to salad leaf and root juices that they can find their way into the plant vasculature during the salad plant’s germination, and once inside, there is no way to wash them out, said Freestone.

 

“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, 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.”

 

Salmonella grows especially well on spinach, said Freestone. “It seems the pathogen prefers spinach.”

 

One safety measure is to eat 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”.

 

Pre-prepared salads are sold increasingly commonly in grocery stores. They also appear in fast food and in airline meals. However, few studies had previously investigated the behaviour of Salmonella within ready-to-eat

 

Safety steps for using bagged salad

 

•        Buy the bag with the best sell-by date

 

•        Avoid lots of damaged leaves

 

•        If the bag is inflated, don’t use it

 

•        Eat as soon as possible after the bag is opened

 

•        Eat on the day you buy it, or

 

•        Remove from bag, remove damaged leaves, wash it and transfer to a sealed container

 

•        Always refrigerate

 

 

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 bagged salad, she said. “We wanted to investigate what happens to Salmonella in a bag of salad to better understand the potential risks to consumers and inform future research on reducing attachment of this pathogen to salad leaves. This study is part of our ongoing research into ways to reduce the risk of Salmonella persisting and growing when it is present in bagged salad.”