If you’re in the mood for salad at your next meal, you may be safer buying your own unpackaged lettuce than you are buying the packaged stuff, a new study finds. Why? Because bagged salad promotes Salmonella growth.
In a new study published in Applied and Environmental Microbiology, researchers from the University of Leicester in England found that juices from cut and damaged leaves not only cause Salmonella enterica bacteria to multiply more rapidly, but also enhance the pathogen’s ability to attach to the leaves of the salad and to the plastic packaging. 
In other words, the juices from the cut salad leaves may help grow more Salmonella bacteria that is more difficult to get rid of. So difficult, in fact, that rewashing the salad to get rid of the pathogen is nearly useless.
Salmonella growth on fresh produce, on the other hand, can be restricted by refrigerating the lettuce. But in the study, researchers found that salmonella still grows faster on refrigerated packaged salad than it does on fresh produce.
The authors of the study write:
“These analyses suggest that Salmonella contaminating a bagged salad would be able to use the leaf nutrients leached into the bag water film to promote its proliferation and retention even within a refrigerated environment.”
The researchers discovered that Salmonella attaches to the lettuce leaves of packaged salads because exposure to the leaves’ liquid aids in the forming of a biofilm that allows the Salmonella to cling more durably to surfaces.
Specifically, when Salmonella bacteria were released into a dish with water and a damaged lettuce leaf, 100 individual pathogens multiplied to more than 100,000 pathogens in the span of 5 days. 
Primrose Freestone, a microbiologist at the University of Leicester, says:
“That’s more than an infectious dose.”
The scientists also found evidence that Salmonella turned more virulent inside packaged produce. A genetic analysis of the bacteria showed they had developed the mutations that would help them to infect people. Freestone says:
I think the bacteria are making a molecular mistake and mistaking chemicals in the salad leaf for ones in the host.”
Dr. Jeri Barak, from the Food Research Institute at the University of Wisconsin-Madison, said:
“It would be fair to conclude that if Salmonella is present in salads, it might grow to infectious doses.
The rates of produce that have been found to be contaminated are between 0-3%. Barak added:
“Consumers should treat bagged salads as temperature-sensitive food products, like milk and ice-cream.”
Could the Packaging Material Promote Salmonella Growth?
Initially, the material used in the plastic bags – polyethylene terephthalate or polypropylene – didn’t show any real tendency for Salmonella to cling to it, but that changed when the juice from the salad leaves was introduced. 
Salmonella bacteria seems to favor spinach, and can infect root juices, allowing it to infiltrate the plant’s insides via the vascular system. 
The study concludes:
“This suggests the salad bag container could be an important bacterial attachment site, even though the salad container has not been factored into considerations of what influences pathogen colonization or retention.” 
In 2010, Consumer Reports tested more than 200 bagged salads to see the level of bacterial contamination. While there were no signs of listeria, E. coli, or salmonella, there were often fairly high levels of bacteria like enterococcus, which are indicators of fecal contamination.
Should Bagged Salads be Avoided Due to Salmonella Contamination?
Does this mean you need to abandon bagged salad entirely? The team says no. Freestone said in a press release:
“Don’t be alarmed, we still eat bagged salad, but don’t keep bagged salad any longer than you need to, we normally buy it on the day we eat it. Buy the bag with the best sell-by date, avoid lots of mushed leaves and if it’s inflated then don’t use it.” 
 BBC News