Listeria is a gram-positive, rod-shaped bacterium that is widespread in nature and capable of growing in both anaerobic (without oxygen) and aerobic (with oxygen) conditions.

Listeriosis is one of the most significant bacterial infections globally, primarily resulting from the consumption of contaminated food. This disease is caused by Listeria monocytogenes, an opportunistic pathogen that predominantly affects individuals with underlying immune conditions, including pregnant women, neonates, and the elderly. Infected individuals may experience severe outcomes such as septicemia, meningitis, or meningoencephalitis. Among the six species of Listeria, only L. monocytogenes poses a risk to human health.

This bacterium thrives at temperatures between 86°F and 98.6°F (30°C and 37°C) but can also grow at temperatures as low as −0.4°C and survive freezing conditions down to −18°C. Its ability to endure such conditions allows thermal characteristics to serve as a distinguishing factor between Listeria and other potentially contaminating bacteria.

Listeria monocytogenes is omnipresent in the environment, often found in water, soil, infected animals, human and animal feces, raw and treated sewage, leafy vegetables, runoff from poultry and meat processing facilities, decaying corn and soybeans, improperly fermented silage, and raw (unpasteurized) milk.

Although foodborne listeriosis is relatively rare, it is a serious disease with high mortality rates, ranging from 20% to 30%, compared to other foodborne pathogens. Severe infections from L. monocytogenes lead to high hospitalization rates (91%) among the most common foodborne pathogens. Additionally, these infections can manifest as sporadic cases or large outbreaks and can persist in food-processing environments, where they can multiply at refrigeration temperatures. This makes L. monocytogenes a significant public health concern.

Ready-to-eat foods are a notable and consistent source of Listeria. A study conducted by the Listeria Study Group revealed that L. monocytogenes was present in at least one food specimen in the refrigerators of 64% of individuals with confirmed Listeria infections (79 out of 123 patients), and in 11% of over 2,000 food specimens analyzed during the study. Furthermore, 33% of the refrigerators surveyed (26 out of 79) contained foods that tested positive for the same strain with which the individual had been infected—a significantly higher frequency than expected by chance.

The potential risk of Listeria in ready-to-eat meats has prompted the USDA to classify this bacterium as an adulterant in these products, leading to a zero-tolerance policy for its presence. The Code of Federal Regulations mandates specific requirements for post-lethality control of Listeria in meat and poultry products, known as “The Listeria Rule,” which was enacted in 2003. This regulation outlines necessary prevention and control measures that food processing facilities must implement to minimize the risk of contamination.