The United States Food and Drug Administration (FDA) admonishes that about 76 million people are sickened by a food-borne illness each year with the hospitalization of more than 300,000 people and about 5000 deaths. Food-borne infections are most common in children younger than four and deadliest in adults fifty and older. Although rarer than infection due to the bacteria Campylobacter jejuni, Salmonella, and Escherichia coli O157:H7, Listeria monocytogenes is the leading cause of death among food-borne pathogens.
The bacterium Listeria monocytogenes can contaminate foods such as unpasteurized milk, soft cheeses made with unpasteurized milk, raw vegetables, and ready-to-eat deli meats. Listeria monocytogenes is a virulent, hard-to-kill pathogen that can survive at refrigeration temperatures; the organism does not form spores, but it can become rooted in food processing environments and persist for a long time, even years. Listeriosis, the food-borne illness caused by Listeria monocytogenes, may present with fairly generic signs and symptoms of gastroenteritis like diarrhea, cramping, and fever. Listeriosis can lead to meningitis, septicemia, and encephalitis in immunocompromised individuals and spontaneous abortion in pregnant women. Listeria infection can be treated with antibiotics, and there are few, if any, drug resistant strains.
Microorganisms are vastly abundant and diverse in the environment, but only a minute proportion is known to cause human disease. Essential questions that researchers are yet trying to answer include: What induces a microbe to become a pathogen? And what are the degrees of separation between a good bug and a bad (disease-causing) bug? For instance, Listeria monocytogenes is ubiquitous in the soil, where it lives off dead or decaying organic matter, e.g., plants. However, upon ingestion by vulnerable humans, the bacterium is capable of transitioning into a well-adapted parasite living in the cytosol of host cells. In fact, the virulence of L. monocytogenes is due, in part, to its ability to survive inside cells of the immune system, then enter and reproduce within different cell types.
The gene products generally expressed at low levels in Listeria’s saprophytic state become elevated in its lifestyle as an intracellular pathogen such that intercellular spread and bacterial replication are promoted. Recent studies indicate that the “outside-to-inside switch” is regulated by a transcription factor known as PrfA. It appears that PrfA activity allows L. monocytogenes to respond to environmental signals and thrive in different ecological settings.
Listeria monocytogenes produces several factors that allow it to invade host cells and grow within them. The pathogen serves as a model system for understanding the infectious-disease process, particularly how an organism can subvert activities of mammalian host cells to its advantage. One of the leading investigators of Listeria pathogenesis is Pascale Cossart, Ph.D., at the Pasteur Institute of Paris and an International Scholar with the Howard Hughes Medical Institute (HHMI). In the April 22nd issue of Nature, Cossart and her team present their recent finding that the secretion of a single toxin by L. monocytogenes inhibits the function of hundreds of host cellular proteins. The team is now trying to determine the identity and role of each of those proteins.
References and Read-more-about-it:
1. Research News; SUMO Wrestlers, Listeria bacteria shut down cellular response to stress. Available at: http://www.hhmi.org/news/cossart20100421.html . Accessed May 2, 2010.
2. Rood JI. Infectious disease: Listeria does it again. Nature. 2010 Apr 22;464(7292):1138-9.
3. Ribet D, Hamon M, Gouin E, Nahori MA, Impens F, Neyret-Kahn H, Gevaert K, Vandekerckhove J, Dejean A, Cossart P. Listeria monocytogenes impairs SUMOylation for efficient infection. Nature. 2010 Apr 22;464(7292):1192-5.
4. Freitag NE, Port GC, Miner MD. Listeria monocytogenes - from saprophyte to intracellular pathogen. Nat Rev Microbiol. 2009 Sep;7(9):623-8. Epub 2009 Aug 3.
5. Freitag NE. From hot dogs to host cells: how the bacterial pathogen Listeria monocytogenes regulates virulence gene expression. Future Microbiol. 2006 Jun;1:89-101.