Where does listeria come from?

Listeria monocytogenes is a bacterium (a type of microorganism), that cannot be seen with the naked eye) and causes listeriosis. It is naturally present nearly everywhere in our environment – it is found in soil and water, rotting vegetation, in our gardens and similar places. It can therefore be expected realistically in/on crops (fruit and vegetables) that are grown in the field and on animals and poultry that are reared in the same environment in which Listeria monocytogenes “lives” naturally.

In very low numbers, Listeria monocytogenesit is not regarded as a threat to human health. It is when levels increase that it becomes a risk to humans, particularly the elderly, infants, pregnant women and those with compromised immune systems or on medication that suppresses the immune system, such as cortisone.

Listeria monocytogenes can contaminate fruit and vegetables, meat and dairy products. That is why hygiene is so important whenever food is stored, processed, sold or prepared.

Unlike many other bacteria that cause disease through food, Listeria thrives in cold conditions, and will even grow (increase in numbers) in the refrigerator. It will not grow in frozen foods and it is killed by cooking and pasteurisation. Pasteurised milk is therefore safe to drink and a thoroughly cooked food is safe to eat. Ready-To-Eat (RTE) foods where there is no further cooking or proper reheating before consumption, are therefore high risk foods for Listeria monocytogenes.

For illness (listeriosis) to occur, the right levels of the bacterium must be present; the strain of the bacterium is also important. Nine different strains have been identified in recent listeriosis patients in South Africa, but the outbreak has been driven by one strain named sequence type 6, or ST6. This is a strain that has also been found in other countries.

In this outbreak in South Africa, Environmental Health Officials have been taking samples of product and of food processing plants and surrounding areas. These tests have found the presence of Listeria monocytogenes in these food processing plants and the vicinity. In fact, it would be surprising if the bacteria were not present, given the fact that they are found almost everywhere, including raw materials such as chicken and other meats. It is important to prevent the organism from contaminating the food product, particularly after it has been cooked.

Food safety for consumers is ensured by the preventative measures that food processing companies take to ensure that Listeria monocytogenes is not found in the food products they produce.

Unfortunately, while South Africa has strict regulations on Salmonella and E. coli, there is no set maximum for the amount of Listeria monocytogenes that may be present in food. There is only a guideline – the South African National Standard (SANS) 885 for processed meats. This guideline is voluntary and is not an enforceable regulation. Many South African food processors therefore seek to abide by international regulations and requirements on Listeria monocytogenes.

Given the recent outbreak of listeriosis, the Department of Health should regard developing appropriate microbiological regulations for processed meats as an urgent priority.

Dr Lucia Anelich