DENVER — Two Colorado farmers whose listeria-contaminated cantaloupes killed 33 people pleaded guilty on Tuesday to federal criminal charges stemming from one of the deadliest outbreaks of food-borne illness in the United States.

Prosecutors last month charged the brothers, the former owners of Colorado-based Jensen Farms, with six counts each of introducing adulterated food into interstate commerce tied to shipping tainted melons to markets.

In May 2011, prosecutors said, the Jensens began washing the farm’s cantaloupes with devices used to clean potatoes and failed to use a chlorine spray feature that kills deadly bacteria.

Both men pleaded guilty in Federal District Court in Denver. They face sentences of up to six years in federal prison and fines of up to $1.5 million for their conviction on all six counts.