Federal regulators say the criminal charges filed against two Colorado farmers over a deadly food-borne illness sends an emphatic message to crack down on food safety.

Cantaloupe farmers Eric and Ryan Jensen were charged in Denver federal court this week with introducing adulterated food into interstate commerce.

Prosecutors say conditions on the Jensens' farm led to a listeria outbreak that killed 33.

The Jensens pleaded not guilty.

The Food and Drug Administration said Friday the criminal case shows farmers must take "absolute care" to keep deadly pathogens out of food.

University of Georgia food safety expert Michael Doyle says improvements are possible. But he says it's tougher to eliminate bacteria from fresh produce because it goes through fewer processing steps than some other foods do.