What is Listeria and how does it cause food poisoning?
Listeria is a gram-positive, rod-shaped bacterium that is ubiquitous and can grow under either anaerobic (without oxygen) or aerobic (with oxygen) conditions.
Listeriosis is one of the most important bacterial infections worldwide that arises mainly from the consumption of contaminated food. The disease is caused by Listeria monocytogenes, which is considered an opportunistic pathogen that affects mainly those with underlying immune conditions, such as pregnant women, neonates, and elders, resulting in septicemia, meningitis, and/or meningoencephalitis.
Of the six species of Listeria, only L. monocytogenes causes disease in humans. Listeria thrives between 86 and 98.6oF (30-37oC), but it can grow at temperatures as low as −0.4°C and survive in freezing conditions down to −18°C. This unique quality allows thermal characteristics to be used as a means of differentiating Listeria from other possibly-contaminating bacteria.
Beyond its ability to grow at low temperatures, Listeria can withstand a broad range of pH, high salinity, and desiccation. Listeria species are also able to adhere tightly to surfaces and form biofilms, which are protective structures of proteins and sugars secreted by the bacteria themselves. These biofilms allow the bacteria to take up residence in a refrigerator, on a preparation table, within a processing line, etc., and persist for months or years, withstanding even some cleaning and sanitizing.
According to a 2014 publication, the World Health Organization noted a global Listeria burden of 23,150 illnesses and 5,463 deaths in 2010. In the United States, there are an estimated 255 deaths due to foodborne Listeria monocytogenes every year, and a case fatality rate of 15-30%. Due to the fatality rate and severe complications, Listeria is considered to be a pathogen of significant public health concern. As one noted expert observed, summarizing the history of these bacteria and their significance for public health:
Although L. monocytogenes was recognized as an animal pathogen over 80 years ago, the first outbreak confirming an indirect transmission from animals to humans was reported only in 1983, in Canada's Maritime provinces. In that outbreak, cabbages, stored in the cold over the winter, were contaminated with Listeria through exposure to infected sheep manure. A subsequent outbreak in California in 1985 confirmed the role of food in disseminating listeriosis. Since then, Listeria has been implicated in many outbreaks of food-borne illness, most commonly from exposure to contaminated dairy products and prepared meat products, including turkey and deli meats, pâté, hot dogs, and seafood and fish.
Because of the prevalence of Listeria in agricultural and food processing and preparation environments, and the severe disease it can cause, many precautions are used throughout the food industry to reduce the risk of illness from this pathogen.