Romaine or cos lettuce is a variety of lettuce that grows in a tall head of sturdy dark green leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat. In North America, romaine is often sold as whole heads or as "hearts" that have had the outer leaves removed and are often packaged together.
Commercially sold romaine lettuce has occasionally been the subject of product warnings by both U.S. and Canadian health authorities warning that consumer supplies can become contaminated with or host pathogenic E. coli bacteria. Cattle can harbor the bacteria without ill effects, and be asymptomatic carriers of the bacterium. Lettuce becomes contaminated with the bacterium as the result of cattle manure being used to fertilize crop fields, or the proximity of cattle pastures and feedlots to water sources used to irrigate crops.