Another outbreak of E. coli in romaine lettuce has sickened more than 100 people nationwide and left others wondering if raw salad is safe.

As food-safety investigators clear the vegetable aisle of contaminated lettuce, authorities provided more guidance on how to shop for safer product.

Consumers should only eat romaine if it is from a harvest region other than Salinas, Calif., or if it was grown indoors, the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) said. If it's not labeled with a harvest region or growing method, don't eat it.

By singling out indoor-grown lettuce as a potentially safer alternative to field-grown lettuce, food-safety regulators are boosting demand for product grown by Medford-based Revol Greens and its greenhouse-growing peers around the country.

"That [government advice] is definitely helping us out this time," said Brendon Krieg, a partner and sales manager at Revol Greens. "We are seeing an uptick in demand from retailers and restaurants because it has such a major impact on their business when they suddenly can't serve salads."

E. coli contamination in produce nearly always comes from irrigation water used on fields, said Kirk Smith, director of the Minnesota Integrated Food Safety Center of Excellence, one of six centers around the U.S. designated by the CDC to strengthen the safety of the nation's food system.

A lettuce field could be contaminated by dust, pests or wild animals carrying E. coli from elsewhere, but it is usually through the irrigation water source "in some way, shape or form," he said.

A major risk with outdoor-grown lettuce is sharing a water source with a nearby animal farm. It's especially risky near cattle, which are widely considered the largest reservoir of E. coli, Smith said.

Some food-safety experts theorize that during California's dry season — which lasts into the fall — the water table drops and the surface water from a cattle operation gets sucked down into the groundwater that is then used to irrigate lettuce crops.

That's why the CDC and FDA are telling consumers to consider buying leafy greens from greenhouse-grown facilities that use alternative water sources.

Revol Greens captures rainwater and snow melt from its greenhouse roof that it then stores in a covered, on-site holding pond. The company tests its water daily and runs it through a chemical-free UV sterilization process to make sure there's no festering bacteria before spraying its indoor lettuce. "Most, if not all, indoor growers of a certain size will have some sort of sterilization for their water," Krieg said.

Symptoms of E. coli usually surface within a few days to a week after ingesting the bacteria and include stomach cramps, diarrhea and vomiting.

Cases in the current outbreak span 23 states, with 31 in Wisconsin — the most of any state. Three cases have been reported in Minnesota. For now, it appears the rate of illness is slowing, with symptoms of the last reported cases beginning Nov. 18. Still, the CDC and FDA recommend consumers avoid purchasing Salinas-grown lettuce for the remainder of this growing season.

A year ago, an E. coli outbreak sickened 88 people in the U.S. and Canada and led regulators to issue a blanket don't-eat-romaine-lettuce warning. Retailers and restaurants pulled all romaine lettuce from shelves and menus before investigators zeroed in on north and central California as the likely source of contamination. But, by then, all romaine growers suffered the consequences.

In response, Revol and five other greenhouse lettuce growers formed a coalition to increase consumer and regulator education, and to encourage the controlled-environment agriculture (CEA) community to adopt stricter standards than already established.

"The coalition was established to develop credible, strong, and appropriate food safety standards, educate consumers and regulators on controlled environment growing, and communicate the value of controlled environment agriculture," said Marni Karlin, executive director of the group, called the CEA Food Safety Coalition.

This year, authorities took a more tailored approach to their warnings, identifying Salinas as the likely growing region in the first public health notice last month. Karlin said the companies she represents were pleased that the CDC and FDA's most recent update called out the relative safety of indoor-grown lettuce.

While most E. coli bacteria are harmless, these investigations track the dangerous types, such as Shiga toxin-producing E. coli, that can be life threatening. Such pathogenic outbreaks have been identified since the mid-1990s with at least one leafy-green outbreak occurring every year since, Smith said.

But what's disconcerting, he said, is how little progress has been made in preventing these outbreaks since the U.S. Department of Agriculture and the FDA gave the industry more serious directives in addressing the problem in the early to mid-2000s.

"This has been a known problem for a long time now, but there are a couple of remarkable things that have happened recently," Smith said. First, spring 2018 was marked by the largest leafy green outbreak of E. coli ever, specifically in romaine, with 219 reported illnesses. The other noteworthy trend, Smith said, is that "we now have recurring outbreaks of the same strain and region."

The reason it keeps happening, he said, is that investigators are rarely able to trace the contamination all the way back to the exact farm.

Lettuce farmers often send their produce to a central processing facility where it is washed and packed. "You could get a bag of lettuce that includes stuff from many different sources," Smith said.

Indoor agriculture is generally much smaller in scale and therefore more easily protected from weather, pests and animals, he said.