There might be more in your salad bowl than lettuce.

Food safety experts say pre-washed salad greens don’t contain dreaded e-coli, salmonella or other food pathogens. But they still might not be all that squeaky clean.

According to a recent study issued by Consumer Reports, many popular brands of prewashed salad greens sometimes contain coliforms [ˈkō-lə-ˌfȯrms] and other bacteria, including enterococcus [ěn’tə-rō-kŏk’əs]. Both these microbes are indicators of fecal contamination.

The organization tested two hundred and eight containers from more than a dozen brands purchased at stores in Connecticut, New Jersey and New York. Thirty-nine percent of the samples exceeded acceptable levels for total coliforms and twenty-three percent exceeded tolerable levels for enterococcus. It made no difference whether the greens were packaged in a hard plastic container. And it didn’t matter whether they were labeled organic or were the baby green variety.

However, certain similarities were noted in the infected samples. Most contained spinach and were tested at one to five days from their use-by date. Packages that still had more than a week to go before their use-by date fared better.

While the source of the contamination was not positively identified, researchers suggested that farm workers often don’t have access to adequate bathroom facilities. They also cited the possible presence of runoff from contaminated streams onto farm fields.

On the basis of the findings, the researchers recommend that consumers purchase their salad greens as far ahead of the use-by date as possible. They also advise thoroughly washing greens even if the bag is labeled “prewashed” or “triple-washed.”

Washing won’t remove all bacteria, they said, but may eliminate residual soil, which can harbor many of the pathogens.