Consumer Reports tests find listeria in fresh greens sold at Costco, Whole Foods

Many people consider a fresh salad a light and healthy option for a summer meal, but new research indicates some salads can make you sick.

Consumer Reports tested 284 samples of fresh greens to find six were tainted with listeria monocytogenes -- a potentially deadly bacteria. The tainted samples included red and green leaf lettuce, spinach and kale. Both conventional and organic salads, as well as packaged and loose, tested positive for the bacteria.

Two of the stores with tainted lettuce included Costco's bagged spinach from Boskovich Farms, which was marked “triple washed," and unbagged green kale from Lancaster Farm at Whole Foods.

Listeria cannot be washed off with water, officials say.

Recall: Sprouts frozen spinach due to possible Listeria contamination

"Washing greens can get rid of dirt and some pesticides, but not all bacteria," said Sana Mujahid, a food safety expert with Consumer Reports. "That's because bacteria can adhere to the surface of leaves and also get stuck in microscopic crevices."

According to scientists, not everyone exposed to listeria gets sick. Some people are more vulnerable, including pregnant women, older adults, infants and young children, and anyone with a compromised immune system. If that's you, Consumer Reports says, carefully consider whether to eat raw greens at all.

"Leafy greens are super nutritious -- and for most people, the nutritional benefits far outweigh the potential contamination risks," Mujahid said. "But if you are in that at-risk category, the safest thing to do is to stick with greens that you can cook."

Recall: Fresh packaged vegetables from Food Lion and Trader Joe's

Experts recommend eating leafy greens soon after you buy them before bacteria has a chance to multiply.

Although Consumer Reports' tests found listeria in the samples, bacteria that could cause other foodborne illnesses, such as salmonella and e-coli, was found in any of the 284 samples tested.

That news is good, but Consumer Reports said more needs to be done to keep fresh greens safe. The FDA said it is currently viewing Consumer Report's data.