Pathogenic bacteria are microorganisms that can cause various diseases in people who come in contact with them. There are many strategies and technologies employed to keep these microorganisms within safe limits.
Among the pathogenic bacteria, we find the genus Listeria, specifically the species Listeria monocytogenes. This bacterium is gram-positive, asporigenic, capsule-free, motile, catalase-positive, and oxidase-negative.
Cooking at temperatures above 65°C inactivates and eliminates Listeria monocytogenes, but recontamination is possible during product packaging. This bacterium tolerates salty environments (up to 25% NaCl) and temperatures between 3°C and 45°C, with an optimum between 30°C and 38°C. It shows good resistance to pH between 4.4 and 9.6, with water activity (aw) of 0.90-0.92, and is very resistant to drying and freezing, which do not affect its viability.
Refrigeration of food can slow its multiplication, but not inhibit it, thus allowing Listeria monocytogenes to contaminate even refrigerated food. This bacterium has low ability to compete with other microorganisms, so it multiplies with difficulty in the presence of other bacteria, but due to its high resistance it can coexist while waiting for better conditions. Being an opportunist, most healthy adults do not experience symptoms after consuming food contaminated with Listeria at low concentrations.
However, categories such as pregnant women, infants, the elderly, and immunocompromised individuals can also develop the disease by ingesting foods with low levels of bacterial load.
Listeria monocytogenes can be present in both raw and processed foods contaminated during or after processing.
Among the foods most at risk are:
- Raw milk and cheeses (including those made from pasteurized milk)
- Seafood and processed fish products (e.g., smoked)
- Meat and animal by-products
- Fresh vegetables
The infection, called listeriosis, manifests with symptoms such as fever, chills, muscle aches, nausea, vomiting and diarrhea. Given its strong resistance and high mortality rates, proper food handling is critical to protect public health. Current regulations impose zero tolerance in ready-to-eat products and very strict limits in all products where this microorganism can be found.
To reduce the risk of contracting the infection in home environments, some simple rules can be followed:
- Complete cooking of food
- Wash vegetables thoroughly before consuming them
- Separate raw meats from vegetables and cooked, ready-to-eat foods
- Consume pasteurized dairy products and avoid cheeses not made with pasteurized milk
- Scrupulously wash hands and kitchen utensils after handling raw foods
- Consume perishable foods quickly.