Listeria is a genus of bacteria with major effects on human health and the environment. Many Listeria bacteria are beneficial, but there's a reason for their fearsome reputation.
The Listeria genus comprises 28 species. Listeria monocytogenes is probably the most familiar, known for causing the foodborne illness listeriosis. Milk pasteurization is important to its control.
Traits of Listeria Bacteria
Listeria are rod-shaped, Gram-positive microorganisms. Facultative anaerobes, they can grow in both aerobic and anaerobic environments.
They are non-spore-forming and distinguished by their ability to move by a process of tumbling motility, which allows them to easily navigate viscous environments like the stomach.
Listeria species are small, comparatively speaking, typically measuring 0.5 to 2 micrometers in length and 0.5 micrometers wide. One micrometer is 0.001 of a millimeter. These bacteria thrive in a wide range of temperatures.
They can survive in environments from just below freezing to normal body temperature (37°C or 98.6 °F) and in some cases up to 50°C (122°F). Refrigeration won't kill them, but cooking at over 65°C (149 °F) does.
They withstand high levels of salt, and acidic environments. This adaptability enables them to survive in some preserved foods.
Listeria is commonly found in soil, water and decaying organic material. It inhabits the gastrointestinal tracts of healthy animals, especially ruminants such as cattle.
This bacterium can contaminate food including dairy products, vegetables, and meat. Farms, food-processing facilities, and food storage sites can be reservoirs for these microbes if not properly monitored.
Listeria bacteria feed on organic compounds in their environment. They are saprophytic, using dead organic matter as a food source. In food, Listeria is most specifically identified in:
Fruits: Cantaloupes and apples have been linked to Listeria outbreaks.
Vegetables: Leafy greens, like spinach and lettuce, can carry these bacteria if contaminated from an outside source.
Dairy: Soft cheeses made from unpasteurized milk are frequent carriers.
Meats: Ready-to-eat deli meats and hot dogs pose significant risks for infection.
In animal hosts, including humans, Listeria can enter the gastrointestinal tract through consumption of contaminated food. The bacteria replicate in host cells, where they evade the immune response.
They can potentially enter the bloodstream. Symptoms range from mildly flu-like to severe infection. The bacteria are later excreted along with other digestive tract contents if the unfortunate host recovers from infection.
Reproduction
Listeria bacteria reproduce asexually through binary fission, dividing once an hour under optimal growth conditions. This rapid replication rate allows them to quickly establish colonies, especially in nutrient-rich conditions.
The bacteria also exhibit intracellular reproduction, allowing them to escape external threats while proliferating within host cells. Rapid multiplication can lead to severe outbreaks.
Significant Species
The best known species of Listeria is Listeria monocytogenes, especially dangerous to pregnant women (causing miscarriage or stillbirth), newborns, the elderly and people with weakened immune systems.
Listeria ivanovii, also pathogenic, is found mainly in sheep. Besides these two strains Listeria bacteria are not harmful. For example, Listeria innocua is often studied in labs to learn more about its nasty relatives.
Impact on Human Health
Listeria monocytogenes has severe health consequences to humans, including meningitis and septicemia, besides listeriosis. The Centers for Disease Control and Prevention (CDC) estimates 1,600 people contract listeriosis each year in the US, with around 260 deaths.
Environmental Health Considerations
From an environmental perspective, Listeria is a threat to livestock and wildlife. As it can persist in agricultural and natural environments, it has the potential to spread among animal populations.
Listeria can also affect environmental health by contaminating soil and water. Their presence in agricultural settings can endanger crops, especially if contaminated animal manure is used as fertilizer.
If infected crops are consumed, Listeria may be transferred back into the human food chain. Their ability to thrive in extreme conditions increases the risk of contamination.
Discovery and Historical Context
The discovery of Listeria dates back to 1926 when it's first isolated by the microbiologist E.G.D. Murray from a rabbit with pneumonia. Considered harmless at first, it soon shows its true nature.
The genus is named after surgeon Joseph Lister (1827 - 1912). Lister pioneers the use of antiseptic techniques in Victorian Era surgery, saving innumerable lives.
Interesting Facts About Listeria
Psychrotrophic Nature: Unlike many pathogens, Listeria can grow at refrigeration temperatures, making it an exceptional survivalist among foodborne pathogens.
Intracellular Pathogen: Listeria can invade and replicate within the cytoplasm of host cells, evading the immune system and complicating treatment.
Extreme Survival Skills: They can survive in extreme conditions, such as high salt concentrations often found in processed foods.
In animals such as cows the disease is most commonly associated with poorly preserved silage with a pH greater than 4.0. Animals may have listeriosis and show no symptoms.
Listeria has the unique ability to cross the blood-brain barrier in humans, which can result in serious neurological damage in those infected.
The first reported listeriosis outbreak is in Canada in 1981, in contaminated hot dogs and deli meats.
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