Food that contains dangerous bacteria is a significant source of illness among people. In serious cases, it can cause chronic disorders and even death. In Norway, we have had major outbreaks of both EHEC (a seriously pathogenic strain of E.coli) and Listeria in recent years.
For food-borne illness to occur, the food must be infected by dangerous bacteria at some stage of the food chain from producer to consumer. An example of this is slaughter, when bacteria can easily be transferred from intestines to carcasses and then on to the processing equipment, which can in turn transfer infection to finished products. It is therefore important to find out the extent to which bacteria are able to establish themselves and survive on surfaces and how they manage this. At the moment, we know little about how bacteria can use each other to achieve this.
Problem bacteria in biofilm
Bacteria are widespread on surfaces. Often different kinds of bacteria can join together, produce a slime and create colonies of bacteria that we call biofilm. Bacteria in biofilms often resist both physical and chemical attack (antibiotics and disinfectants for example). Bacteria in biofilms can therefore be particularly problematic. We know that individual strains of bacteria have different abilities to adhere and create biofilm, but bacteria are seldom found alone. Few studies have looked at how biofilm is created under more realistic conditions, that is to say when different bacteria occur together.
Biofilm has been investigated more closely in a research project at Nofima Mat. The study shows that there is a complicated interaction in which bacteria can make use of each other's properties, so as to be able to establish themselves on surfaces. Bacteria that are considered to be poor at creating biofilm can establish themselves, create biofilm and represent an increased risk of infection in conditions where other bacteria are present.
To find out which bacteria have adapted to the production environment, samples were collected from the equipment and premises of two meat companies. The bacteria could be identified by sequencing their DNA. The analyses showed that most of the bacteria that survive cleaning are not dangerous to people, but they may have a significant effect on food quality. Some of the bacteria had the ability to survive in a dry environment, while others were good at creating biofilm on surfaces in a moist environment. Pathogenic bacteria such as certain strains of E.coli on the other hand are known as relatively poor creators of biofilm in the conditions found in meat processing companies.
By using various microscopy and microbiological techniques, we found that a bacteria from a slaughterhouse, such as Acinetobacter calcoaceticus, which is good at creating biofilm can help E.coli to establish itself in biofilm and thereby survive in the environment.
Helped E. coli to create biofilm
By using microscopy we were able to obtain a visible understanding of how the bacteria adhere and the biofilm is created over the course of time. The trials were done under static conditions and also under conditions with constant renewal of the supply of nutrient medium(food for the bacteria). In the latter conditions, A. calcoaceticus was shown to have a significant effect on the establishment of E.coli in biofilm. Here the E.coli was unable to create biofilm when alone. When E.coli was present together with A. calcoaceticus, firstly the A. calcoaceticus created a biofilm. This biofilm had a fungus-like structure and it was found that E.coli established itself in this structure and was gradually covered by A. calcoaceticus cells (see illustration).
Tip of the iceberg
It has often been claimed that pathogenic bacteria flourish in surroundings that are "too clean", but this study shows that it may be environmental bacteria that survive cleaning and protect pathogenic bacteria. This is relevant for the food industry, where the production of safe and long-lasting food is essential, but also, for example, for medicine, where bacteria and biofilms on surfaces are a significant cause of hospital infections.
In the future, knowledge about which bacteria promote the establishment of pathogenic bacteria in biofilms can be actively used in combating these pathogenic bacteria. This study illustrates only a small step along the way to a better understanding of the survival of unwanted bacteria on surfaces.
The research is financed by the Research Council of Norway, the Foundation for Research Levy on and Agricultural Products and the EU ProSafeBeef project.