Listeria

Listeria is a gram-positive rod-shaped bacterium that can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions. Of the six species of Listeria, only L. monocytogenes causes disease in humans. These bacteria multiply best at 30-37^oC, but also multiply better than all other bacteria at refrigerator temperatures, something that allows temperature to be used as a means of differentiating Listeria from other contaminating bacteria.

Called an “opportunistic pathogen", Listeria is noted to cause many cases per year of severe invasive illness. Perhaps not surprisingly then, "foodborne illness caused" by Listeria monocytogenes has raised significant public health concern in the United States, Europe, and other areas of the world.