Pre-washed salads may not be as clean as they appear, according to recent research.

US researchers found high levels of bacteria linked to faecal contamination in a number of bagged salads.

They said that there was a higher risk of E coli and salmonella contamination than people realised when they made purchases.

However, the researchers also said that the contaminating bacteria were not something  to worry about on the whole.

E coli contamination in pre-washed salads killed three and hospitalised more than 100 sufferers about three years ago in the US.

Investigators never found the root of the contamination, although they suspected that the bacteria had come from livestock.

For the purposes of the recent study, researchers examined 208 packaged salads from 16 different brands.

All of the salads were purchased in the eastern US, and all of them were packaged in plastic.

The researchers found that 39% of the 208 study samples probably had more than 10,000 faecal bacteria per gram.

And 23% of the 208 samples had more than 10,000 enterococcus bacteria per gram.

Enterococcus bacteria are strictly monitored in bodies of water where people swim, and exposure to them can cause urinary tract infections, meningitis, and other problems.

In general, mixes of spinach with other leaves had the highest levels of bacteria. Packaging had no effect, in general, upon the levels of bacteria in the salads.

While some pre-washed salads only contained small amounts of bacteria, others contained up to 1 million colony forming units (CFUs).

Michael Hansen, senior scientist for Consumer's Union in the US, said that shoppers should try to buy pre-washed salads when the salads were at least six days from going off.

He said that all pre-washed salads should be re-washed before people ate them.

In a joint written statement, the US Produce Marketing Association and the United Fresh Produce Association said that their industry growers had already invested a lot of money in food safety research programmes.