Listeria and Food Poisoning

One of the most important issues to consider as a food manager concerns listeria and food poisoning. It has a reputation as being a tricky bug to control and, for that reason, Listeria species are used as an important benchmark for the microbial quality of food by food examiners who will also look for an absence of the pathogen Listeria monocytogenes.

Listeria outbreaks were normally associated with meat products and unpasteurised cheeses; in fact one of the most deadly food poisoning outbreaks occurred in South Africa recently involving a processed meat product and was said to claim the lives of 216 people. More recently though, listeria has also been isolated in other foods such as frozen fruit and vegetables and pre-packed salads.

Listeria – Character and Control

Like many bacteria, thorough cooking will kill the organism and ensure food safety. It is, however, more difficult to control listeria in raw or ready-to-eat foods as the organism is able to grow at low temperatures during storage. Although, strict temperature controls will reduce microbial growth (processed products will tend to present less of an issue if they have a low pH and/or have a lower water activity). Foods that are not subject to heat treatment have the possibility of being contaminated. Contamination during preparation or after the cooking stage is likely to be one of the other reasons for listeria presence. It is therefore important that manufacturers have rigorous disinfection controls in place and ensure that personnel avoid food or environmental contamination. Minimising the shelf-life of food after processing will also minimise growth.

Raw foods should be separated from ready-to-eat foods, particularly meat and poultry. However, vegetables and fruit may also be contaminated as the bacterium is found in the environment and soil. All fruit and vegetables should be washed thoroughly and any visible physical contamination removed. Peeling root vegetables will help remove surface contamination prior to washing. Separation of tasks and proper cleaning and disinfection will help prevent cross-contamination. Once introduced to the food processing environment Listeria species can be very stubborn.

Listeriosis – Listeria and Food Poisoning

People usually contract listeriosis after having consumed food contaminated with the bacteria listeria monocytogenes. There is quite a long ranged incubation period of 7 to 70 days (the time it takes before symptoms start). Symptoms range from mild nausea, flu-like symptoms and diarrheoa to headaches, convulsions and loss of balance if the bacteria moves on from the gut.

Depending upon where you are in the world the fatality rate varies. In some countries the fatality rate is around 20% whereas others this may rise to 30% ; the variance depends upon factors such as quality of healthcare and speed of detection.

The very young, very old or immuno-compromised are most susceptible to fatality from listeriosis. Illness during pregnancy also carries a risk of miscarriage or stillbirth which is why pregnant women are warned to stay away from certain foods, such as unpasteurised cheeses, smoked fish and pates. They are also advised to stay away from farm animals, particularly during lambing season as close contact with animals does present a significant risk.