Prepackaged salads may promote the growth of salmonella bacteria, researchers report.

They found that even slight damage to leaves in salad bags released juices that encouraged the spread of salmonella.

These salad juices also boosted the bacteria's ability to form biofilms, which cling tightly to the surfaces they coat. This makes it hard to wash the bacteria off the produce, the researchers said.

Most salad leaf crops are first exposed to salmonella in the field, from sources such as insects, bird droppings and manure, explained study co-author Primrose Freestone, an associate professor of clinical microbiology at the University of Leicester in England.

She noted that prepackaged salads are common in grocery stores, and are also served in fast food and airline meals, but few studies had examined how salmonella behaves in these products.

Her team's study was published Nov. 18 in the journal Applied and Environmental Microbiology.

"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," Freestone said in a journal news release.

"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," she said.