Co-2 cells or islets of undifferentiated Caco-2 cells (332, 333). Listeria. Listeria monocytogenes is a ubiquitous, Gram-positive bacterium that thrives in diverse environments such as soil, water, various food products, human beings, and animals (300, 301, 334). The disease caused by this bacterium, listeriosis, is acquiredby ingesting contaminated food products and mainly affects immunocompromis