LISTERIA AND BIOFILM

THE PREVENTION IN FOOD INDUSTRY

Listeriosis is an infection caused by the bacterium Listeria monocytogenes, a microorganism with a natural habitat in soils, plants and water. If ingested in large quantities through contaminated food, this bacterium can give very serious consequences on humans.

It is therefore essential to prevent the contamination of food surfaces by using specific cleaners and by following accurate cleaning procedures that can be easily adopted by any food and beverage producer.

The upgrowing restrictions on this issue worldwide

Although the number of cases of listeriosis reported worldwide is relatively small to date (a bit more than 2,000 in Europe in 2021), there is evidence of a high rate of hospitalizations and deaths caused by this infection.

This pathogen is particularly dangerous if ingested by people with weak immune defenses; a fact that is worrying if we consider the constant rise in average aging in developed countries.

For this reason, national legislations have set limits to the count of Listeria monocytogenes in food and beverages outcoming from production plants. In some countries, such as the United States of America, there is no tolerance for the presence of this microorganism. Of course, the limits imposed in the United States are also valid for producers of other countries who want to export in this country, and therefore the organizations which authorize these movements of goods put a lot of pressure in order to have foodstuff totally free from Listeria.

How listeria contamination occurs

As it lives on the grounds and in surface waters, it is with raw materials that Listeria enters food plants. It is a microorganism that does not have great resistance to high temperatures or other antimicrobial treatments; however, it tolerates high saline levels and is able of multiplying even at low temperatures. Moreover, in the last years, it has been known that Listeria is not only capable of surviving inside biofilms formed by other microorganisms, but it is itself a biofilm promoter.

Protected by biofilm, it becomes little sensitive to disinfection treatments and it multiplies undisturbed continuing to contaminate food and beverages with which it comes in contact.

Until recently, it was thought that contamination occurred only from "mature" biofilms, i.e. biofilms having such a dimension to be "damaged" by external flows, or from which microorganisms move away because they can no longer find feed inside.

But recent papers have shown the dynamism of the relationship between biofilm and external environment with the consequent possibility of continuous contamination of food: this has changed the approach to the prevention of contamination by Listeria, and more generally by biofilm, which until then was mainly oriented to prevent the maturation of the latter, using enzymatic systems before it reached a critical size.

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The prevention in food industry

The relevance of acting on the first phase of biofilm development, i.e. the initial phase of adhesion of promoter microorganisms to surfaces, has determined important innovations in the prevention of foodstuff contamination.

Today we know that adhesion is favored by the presence of organic and inorganic residues: in particular, the presence of calcium ions is crucial for the microbial structures involved in anchoring to the surfaces.

To summarize, it is important to consider these factors when drawing sanitation procedures to be adopted in the production plants:

The microorganisms can anchor themselves to surfaces only if on them there are traces of organic matter and mineral cations that allow anchorage and protection.

Only adequately sequestered and surfactant-activated cleaning solutions avoid the deposits listed above and thus prevent the first phase of biofilm formation.

Washes carried out with simply caustic solutions (e.g. only soda) do not have the ability to remove inorganic residues from surfaces.

These concepts derive from the universally valid laws of thermodynamics.

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