Listeria monocytogenes

Listeria monocytogenes on blood agar

Listeria monocytogenes, or just “Listeria” for short, is the causative agent of the food-borne illness known as Listeriosis. It is one of eleven organisms in the Listeria genus.

L. monocytogenes is a motile, non-spore-forming, gram-positive bacilli that can grow in low temperatures, such as those in the refrigerator. It is facultatively anaerobic. On blood agar listeria exhibits slight beta hemolysis, and a motility test will reveal a distinct umbrella pattern at room temperature. Listeria is highly motile at room temperature thanks to its set of peritrichous flagella (which do not appear at 37 C). Inside of the body, listeria produces the exotoxin known as Listeriolysin O, which is one of three virulence factors that allows it to escape phagocytosis. If the listeria manages to become phagocytized by host cells, however, it becomes blood-borne and can result in septicemia as well.

Listeria monocytogenes false-color electron microscope image showing peritrichous flagella

Although listeria is considered uncommon and usually self-limiting, it is still the third most common enteric pathogen, behind Salmonella and Shigella. It is an opportunistic pathogen, and the sequelae of untreated infections can be numerous. The bacteria can even survive refrigeration, which makes soft cheeses and lunch meats particularly vulnerable breeding grounds for this pathogen. Groups considered most at-risk are pregnant women, children, and the elderly. Listeria can be particularly dangerous for pregnant women, where listeriosis can result in stillbirth, spontaneous abortion, or meningitis of the infant. Deaths from listeria outpace fatalities from Salmonella and botulism (Clostridium botulinum). Listeriosis is fatal in approximately 20% of cases, which makes it a relatively rare but serious illness to contract.