Listeria
Listeria refers to a genus of bacteria, with Listeria monocytogenes being the primary species of concern due to its role as a foodborne pathogen. It is commonly found in environments such as soil, water, and decaying vegetation, and can contaminate a variety of foods, including vegetables, poultry, and soft cheeses. Ingesting contaminated food can lead to listeriosis, a serious illness that can cause severe health complications such as meningitis, sepsis, and abortion in pregnant women. Although many individuals infected with L. monocytogenes may remain asymptomatic, the disease poses a significant risk to vulnerable populations, including infants, the elderly, and those with compromised immune systems.
Listeria monocytogenes is particularly resilient, able to grow in refrigerated conditions and withstand acidic environments, which increases the likelihood of food contamination. While antibiotic treatments like ampicillin can effectively combat infections if administered early, there are emerging resistant strains, and currently, no vaccine exists to prevent listeriosis. The incidence of listeriosis is relatively low, with severe outcomes reported in a small number of cases worldwide each year. As a significant public health concern, understanding Listeria and its effects is crucial for food safety and disease prevention.
Listeria
- TRANSMISSION ROUTE: Ingestion
Definition
The bacterium Listeria monocytogenes commonly contaminates foods. It is difficult to detect and, in rare cases, causes listeriosis, a serious and often fatal disease in humans.
Natural Habitat and Features
Listeria species are found in soil, decaying vegetation, water, sewage, and field crops such as alfalfa. The only pathogenic species, L. monocytogenes, occurs in humans, other mammals, birds, fish, crustaceans, and insects. L. monocytogenes also occurs in foods such as vegetables, poultry, fresh and processed meats (such as bologna and hot dogs), soft cheeses (such as feta, farmer’s cheese, and queso blanco), and salad dressings. It occurs in raw foods and processed products. The first three confirmed outbreaks of listeriosis, in the 1980s, were reported from ingesting coleslaw, milk, and Mexican-style cheese (queso fresco), respectively.
Listeria spp. are characterized as gram-positive, non-spore-forming rods, which may occur in short chains or coccoid forms. They are facultative anaerobes, are catalase-positive, always ferment, and ferment sugars to produce acid. Their cell walls contain teichoic acids but not mycolic acids. An unusual property of L. monocytogenes is that it can be intracellular in animal cells.
L. monocytogenes grows slowly under refrigerated conditions and is resistant to cold, acid, and salt. These properties increase the risk of infection from ingesting processed and stored foods that may be contaminated.
L. innocua has a significantly larger genome than L. monocytogenes (3.01 megabases compared with 2.94 megabases), though L. innocua lacks some virulence genes and is nonpathogenic. Both genomes code for about 3,000 proteins, with perhaps 100 to 120 more in L. innocua. Thus, L. innocua grows in a wide variety of environments and is easier than L. monocytogenes to detect. Because of many genetic and metabolic similarities and because they are often found together in a variety of situations, L. innocua could be used as an indicator of L. monocytogenes in quality control and regulatory analyses of foods. The two species grow slowly and indistinguishably on many laboratory media but can be distinguished by PCR (polymerase chain reaction) and colony morphology on special blood agars. Pulse-field gel electrophoresis of genomic deoxyribonucleic acid (DNA) and ribotyping are also used to distinguish Listeria spp.
Of the thirteen serovars of L. monocytogenes, listeriosis is most often caused by strains of just three: serovars 4b, 1/2a, and 1/2b. In the lab, serovars are distinguished by serotyping and phage typing.
Pathogenicity and Clinical Significance
Even as a food-borne disease, listeriosis is not mainly a gastrointestinal illness. Instead, severe symptoms include meningitis, sepsis, and abortion in women in late-term pregnancy. Many people who ingest L. monocytogenes do not show any symptoms, but they may carry bacteria and act as a source of food contamination. The disease is especially risky for infants, older individuals and bedridden, and persons with acquired immunodeficiency syndrome (AIDS) or other immune disorders. Incidence of listeriosis is about 0.1 to ten cases per one million people per year worldwide. Case fatality is quite severe at 20 to 30 percent. Thus, the United States reports about 260 deaths per year from listeriosis. Most cases are sporadic, and epidemics are rare.
The virulence of L. monocytogenes is an inherent property of the bacterium because of virulence genes carried in its genome. After entering the gastrointestinal tract, bacteria are attacked by phagocytes but resist destruction. Instead, they persist intracellularly, proliferate inside phagocytes, and release progeny when phagocytes lyse, or break apart. The bacterial pore-forming protein listeriolysin O facilitates cell lysis of phagocytic cells. The bacterial surface protein internalin (InlA) aids in the crossing of the intestinal barrier to enter the nervous system. Other bacterial virulence proteins are InlB, ActA, PlcA, and PlcB. These proteins (with InlA and listeriolysin O) are encoded by virulence genes, some linked in a 10-kilobase region of the L. monocytogenes genome, absent from L. innocua.
Drug Susceptibility
Listeria spp. are susceptible to many different antibiotics, though resistant strains are emerging. L. monocytogenes infection in humans can usually be treated with ampicillin, especially if treatment starts early. No vaccine is available.
Bibliography
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