Listeria monocytogenes is a Gram-positive, short, motile, non-spore-forming, beta-hemolytic (breaks down red blood cells) bacillus that does not produce any specific toxin. The organism is capable of growing at temperatures of 0°C (32°F) and lower. Of the four species in the genus Listeria, only L. monocytogenes is pathogenic for humans. Human listeriosis is much more common and just as serious as diphtheria, but it is not so easily recognized since it rarely occurs in an epidemic fashion. Listeria infection is serious enough that 92% require hospitalization, where 20% will die. Listeria thrives in such places as on the inside walls of refrigerators, where it can grow from a few cells to millions in just a few weeks.
Most infections are seen as neonatal sepsis or meningitis, spontaneous abortions or stillbirths, and as sepsis or meningitis in immunocompromised patients. Of these, meningitis is the most common presentation of the disease. Pregnant women are most at risk since the organism can cross the placental barrier into the developing fetus. Pregnant women who have persistant flu-like symptoms or fever of unknown origin should be examined for possible listeriosis.
Listeria monocytogenes is ubiquitous (found everywhere), occurring in soils and plants, as well as in sick and well humans. It is found naturally in the intestines of humans, birds, spiders, livestock, and other animals. Most infections begin with the ingestion of contaminated animal foods or their products. In one instance alone, in the year 2000, Cargill Turkey Products recalled seven million pounds of their products believed to be contaminated with Listeria (Robbins). It is not clear why some persons develop the disease while others do not, but there is a clear link to increased risk in those whose immune systems are already compromised. Penicillin is the drug of choice for treatment.
Since it can thrive on refrigerated foods, it is often isolated from delis and buffets. About half of all cook-chill foods contain the organism, where it recovers from heat damage during the chilling process. As well as thriving in cold temperatures, it lasts for long periods of time – at least eight hours – on the hands, and is not easily removed by conventional handwashing. Foods associated with this microbe are soft cheeses, pates, and raw vegetable dishes. It is the microbe that is the most common cause of death from food contamination.