DENVER – Criminal charges against two cantaloupe farmers over a deadly food-borne illness send an emphatic message to fruit and vegetable growers to crack down on safety, federal regulators said Friday.

Colorado farmers Eric and Ryan Jensen appeared in shackles in a Denver federal court this week and pleaded not guilty to charges of introducing adulterated food into interstate commerce.

The federal Food and Drug Administration has said conditions at Jensen Farms in southeast Colorado led to a 2011 listeria outbreak that killed 33 people. Officials said people in 28 states ate the contaminated fruit, and 147 required hospitalization.

The criminal prosecution “sends the message that absolute care must be taken to ensure that deadly pathogens do not enter our food supply chain,” the FDA said in a statement Friday.

RARE CRIMINAL CHARGES

Criminal charges are rare in food-borne illnesses, but the FDA under President Obama has been more aggressive in pursuing farmers and food processors for alleged lapses, said Michael Doyle, director of University of Georgia’s Center for Food Safety.

“I think the FDA is sending a strong message that the produce industry is going to have to raise the bar to ensure the safety of the, basically, ready-to-consume foods,” he said.

It’s the second such warning from the agency, Doyle said. In February, four former employees of a peanut company were charged in Georgia federal court with scheming to manufacture and ship tainted peanuts. A 2009 salmonella outbreak blamed on the peanuts killed nine people and sickened hundreds.

The four pleaded not guilty.

The listeria epidemic traced to Jensen Farms was the nation’s deadliest outbreak of foodborne illness in 25 years.

DIRTY WATER, EQUIPMENT BLAMED

The FDA concluded the melons likely were contaminated in Jensen Farms’ packing house. It said dirty water on a floor, and old, hard-to-clean equipment probably were to blame.

The Jensens’ trial is scheduled to start Dec. 1. The brothers could face up to six years in prison and $1.5 million in fines each if convicted.

Produce farmers don’t have a “true-kill” step to eliminate bacteria, the way dairies and other food producers do, Doyle said.

Pasteurization and proper storage can assure the safety of milk, he said. “We cannot say that with bag salads because we don’t have that true-kill step that will kill harmful bacteria,” he said.