Bacterial growth in food is a major risk issue, with large consequences on the health, safety, environment and economy of food consumers
and manufacturers. Minimally processed food is becoming increasingly popular in Europe, but it is susceptible to bacterial growth, including
the human pathogen Listeria monocytogenes (LM). Various treatments exist, including thermal treatment, salt additives, etc., but not all are
suitable to purge fresh food from bacteria, due to considerations of taste, texture, quality and health (such as reducing salt concentrations).
High pressure processing (HPP) is a promising method as it can kill most bacteria with no adverse sensory side effects on the quality of
many food product groups. However, it has a limited success, since some bacteria, including LM, manages to recover rapidly from high
pressure treatment, increasing the risk of food contamination.Until recently, there were only speculations of the reasons for this fast recovery after the high pressure treatment.
This transnational SafeFood industrial biotechnology project united 8 groups from 6 countries across Europe, with the purpose to
turn food safer, by killing the LM. The consortium studied several mechanisms that allow LM to survive HPP.
The consortium found several important results that can change the manner we process the food in the industry.
Firstly, it was found that although the high pressure processing kills many bacteria cells, this processing method does not purge all the bacteria completely. Our findings suggest that bacteria that appear to be dead, are actually inactivated but not dead. We show that by showing that their genetic material, their RNA, that is degraded in dead cells after minutes, is stable even after several days, and in mobilized to enable functionality. This is published in the open access scientific channel BMC Genomic (2020).
Secondly, it appears that some bacteria has ability to survive the shock by evolutionary mechanisms. The consortium published in scientific open access publication channels (e.g. BMC genomics) that the bacteria undergoes transition from SOS response to the shock, to recovery mode during the first 2 days, and after a process of repair, including of the membrane, and finally return to functional division after several days. Moreover, by special scientific tools we used (fluorescence microscopy) and modeling, we managed to calculate the size of the pores, damage holes, in the membrane of the bacteria resulting from the high pressure processing. For the first time in history, people could observe how holes in the membrane of the pathogen listeria monocytogenes are recovered during days. Additionally, we investigated extensively the genetic changes that happens in the cells that are alive after the high pressure processing, and managed to better understand how they survive, how they mobilize the genes and proteins to survive, and to repair their damage.
The implication of these results are that the cells survive exceptionally well, also to very high pressure values that are applied in the industry. We found that the high pressure processing does not kill all the bacteria population in the food, and the surviving ones manage to recover by a special recovery modes the stems from evolution. It implies that the 'zero listeria' demands from authorities on import of food (for example USA's demands for zero listeria) is not realistic, since the bacteria is inactivated, rather than dead. By understanding the mechanisms for this ability to survive, we will later learn how to target specific protein to prevent the bacteria from repairing their membrane and survive. This have the potential to extend the shelf life of the food significantly weeks, and even months.
1. Wider knowledge of how the pathogen Listeria survives and grow under different conditions. Novel knowledge about its temporal gene mechanisms that may help the industry to target and deactivate.
2. Our results can help to improve the high pressure processing regimes in the industry. for instance by increasing the holding times and decreasing the pressure. This will improve the economy of the HPP and bacteria treatment of food.
3. Our results has the potential to increase the shelf life of the food, by preventing or delaying the growth of listeria in the food. This has both economic impact, but also environmental impact.
4. Listeria is deactivated, but not dead after HPP. This implies that the demands (for instance by the USA authorities) for 'zero listeria' is not realistic one, because although measurements indicate zero growth in the first days after HPP, it does not imply zero listeria, as the pathogen is still presence and alive in the food.
Fresh and ready-to-eat (RTE) foods are becoming increasingly popular in Europe. Food, particularly RTE, is susceptible to foodborne pathogens such as Listeria monocytogenes (LM). The LM is the cause of listeriosis, a disease with increasingly health and economical issues in Europe. Food manufacturers must comply with strict EU regulations of LM in food, whereas the US adopted a zero-listeria policy. This poses also great economical and environmental challenges, due to large food waste and short expiration dates. Various ways for mild conservation exist, including mild heat, salt, and pH adjustments. However, not all treatments are of interest due to undesired changes in taste, texture and quality. Traditional thermal and preservative conservation can have negative health issues and often lead to reduced nutritional quality of foods.
High pressure processing (HPP) is a new and promising method. It has reached increasing popularity as it can prolong shelf life and preserve the quality of the food. However, LM is highly durable and some survive the HPP treatment by having fast recovery. They are even able to grow in refrigeration. Recent studies show that stress induced by HPP damages primarily the cell walls, cell membrane and DNA structures. The bacteria respond by activating regulatory genes and pathways that are responsible for the expression of specific repair enzymes, repairing the damages in a very efficient manner, thereby preventing cell death. Targeting these regulatory genes and pathways will provide an excellent foundation for developing SafeFood, an improved HPP method which can fight foodborne bacteria.
This transnational SafeFood industrial biotechnology project unites 8 groups from 6 countries across Europe, managed by the
Norwegian University of Science and Technology (NTNU), with the purpose to turn food safe, by killing the contaminators LM.