Listeria monocytogenes is a Gram-positive, rod-shaped, bacterium – microscopic in appearance, but looming large among public health officials for its ability to result in fatalities.
First observed in the scientific literature in the 1920s, it’s been widely known among microbiologists since the 1960s. This stealthy organism is ubiquitous in the environment, often found in moist conditions, including soil, surface water and decaying vegetation. Salt-tolerant and able to both withstand and grow in temperatures below 1°C, L. monocytogenes is a hardy pathogenic organism. Its flagella – hair-like appendages protruding out of its cell structure – allow it to swim through a variety of hosts, though at a warmer and narrower temperature band.
Listeria monocytogenes is the most infamous of the seventeen known species in the Listeria genus. Only one other member of its tribe (Listeria ivanovii) is considered pathogenic, but it mainly resides in animals, not humans.
“L. mono” is not a leading cause of foodborne illness, however, it is among the leading causes of death from foodborne illness. The U.S. Centres for Disease Control and Prevention (CDC) estimates that, in the U.S., domestically acquired foodborne L. monocytogenes results in 255 fatalities per year. These numbers are down from figures seen prior to the turn of the century when the CDC estimated approximately 500 such deaths in the United States annually.
Persons most affected by L. monocytogenes’ bacterial infection, listeriosis, are those with immune system deficiencies, caused by anticancer drugs, AIDS and other reasons. Pregnant women are disproportionately infected with L. monocytogenes, but they usually only experience mild, flu-like symptoms. Sadly, the babies they’re carrying don’t always fare as well – passing away in utero or born stillborn at concerning rates.
Overall, the most severe form of infection has a case-fatality rate of 15 to 30 percent of the population. When listerial meningitis occurs, the case-fatality rate may be as high as 70 percent. In perinatal/neonatal infections, it’s more than 80 percent.
Many foods have been associated with L. monocytogenes, especially when they’re in raw form. Prime examples include ready-to-eat processed products, poultry and meats, vegetables, seafood and improperly or unpasteurized milk, cheeses (particularly soft varieties) and ice cream.
Contamination can come from these raw materials, from the air and from food workers and food-processing environments. Most experts agree that post-processing contamination from food-contact surfaces represents the clearest and most present danger, and surfaces can harbor the organism for years. For example, the identical strain of L. monocytogenes collected from a human listeriosis case in 1989 was isolated more than a decade later from a fresh slice of turkey produced by the very same processing plant.
Several high-profile L. monocytogenes outbreaks over the past decade have plagued food companies and their consumers, drawing more concern and attention from inspectors, the media and the general public. Some have dubbed the swelling interest in this pathogen as a “Listeria hysteria.”
Certainly, control of Listeria species, including Listeria monocytogenes, is vital. Food processors mustn’t let their guard down to test for this persistent pathogen, methodically sampling end product and key areas within food production environments as well as other food contact and non-food contact surfaces in plants that could be reservoirs for Listeria. They also must be mindful of the fact that L. monocytogenes can grow in refrigerated temperatures.
To assist these critical efforts, food producers count on technology to potentially grow selective organisms like Listeria monocytogenes to detectable levels while simultaneously preventing less harmful microorganisms from also growing and interfering with accurate testing. Then they must evaluate its presence and prevalence.
Rapid-test kits for Listeria species as well as the Listeria monocytogenes pathogen have been among the fastest-growing tests in the food industry. As global regulatory requirements have changed and become more stringent, the demand for Listeria testing has dramatically increased. In addition, as the food industry has become more global and increasingly competitive, processors want results that can be delivered faster and with more details.