A UK study shows small amounts of damage to packaged salad leaves significantly increases the amount of the harmful salmonella bacteria.

Ready-to-eat bagged salads can be a breading ground for harmful salmonella bacteria, a UK study has found.

Salmonella grows especially well on bagged spinach, says Dr Primrose Freestone, an expert in clinical microbiology at the University of Leicester.

Salmonella poisoning is usually linked to consuming inadequately cooked meats or poultry.

However, salad leaves can become contaminated with salmonella by coming into contact with dirty water, direct contact with fertiliser, pests or animals in the field, or if they are not properly washed before sale.

With pre-prepared salads increasingly being sold in supermarkets, a new study published in the journal Applied and Environmental Microbiology has found they can potentially pose severe health risks.

The University of Leicester study found just small amounts of damage to leaves in bagged salads already contaminated with Salmonella enterica encourage the growth of the bacteria.

Using salad juices in water to mimic the grocery salad bag environment, the scientists showed movement of individual salmonella bacteria more than doubled in such an environment.

This increased movement significantly aided bacteria growth.

After five days of refrigeration 100 salmonella bacteria had multiplied to about 100,000.

It was also found the salad juices boosted the formation of "biofilms" on the salad leaves, which are very resistant to being washed off.

Salad leaf crops are usually grown in open fields where they can first become contaminated with salmonella via insects, bird faeces and manure.

While uncommon, salad leaf contamination can occur and good hygiene and washing is important to prevent a salmonella outbreak, Dr Freestone says.

Each year an estimated one million Australians have to visit a doctor with food poisoning, about 32,000 end up in hospital and 86 people die.

The Food Safety Information Council says one way of lowering the risk of salmonella is ensuring salad leaves are washed before consumption.

Consumers should also ensure products past their use-by date are not eaten because this can increase the risk of contamination.