Ready-to-eat spinach bears tough microbes.

People who eat bagged salad may be getting more than they bargained for. Researchers have found that some ready-to-eat spinach contains a significant number of bacteria, many of which are resistant to several antibiotics.

The popularity of convenience foods, such as prewashed salads, has surged. "People assume that 'ready-to-eat' means that it's clean" and relatively free of bacteria, says Sonia Walia of Oakland University in Rochester Hills, Mich. She and her colleagues set out to test that assumption by investigating several bags of Dole Ready-To-Eat Baby Spinach, a popular bagged salad.

The researchers pureed the spinach from each bag in a sterile mixer, then spread spinach extracts onto petri dishes containing nutrients that encourage bacterial growth. Within days, they had identified several types of bacteria living in the spinach mixture, including Staphylococcus, Enterobacter, and Escherichia species, which can infect people. By challenging these bacterial cultures with some common antibiotics, such as ampicillin and ciprofloxacin, Walia's team found that about 95 percent of the colonies they tested were resistant to two or more antibiotics.

Walia and her colleagues plan to perform similar studies on other types of bagged salads, such as iceberg lettuce, as well as on loose produce, such as spinach found in vegetable bins. They suggest that consumers can avoid eating antibiotic-resistant bacteria in bagged spinach by washing it, even if it's labeled as pre-washed, and by cooking it.--C. B.