Food scientists have developed a diverse toolkit to help food producers keep pathogens out of fermented foods, including pasteurization, raw ingredient and end-product testing, and the addition of natural preservatives. Over the past decade, a new technology using viruses that attack pathogenic bacteria (bacteriophages), has emerged on the market as a potential addition to this food safety toolkit. Here we explain the science of using good viruses to kill bad bacteria in fermented foods.
What is a bacteriophage?
A bacteriophage is a virus that attacks bacteria. Phage is another term for virus. Just like the viruses that attack humans and plants, these viruses are relatively simple and have a minimal number of biological parts. These are viruses that are specific to bacteria; they cannot infect humans because they don’t have the machinery to get inside and persist inside of the cells that make up your body. They can only make more copies of themselves when there is a living bacterium (a host) present to infect. Not all bacteria can serve as hosts to all phages; there is some level of specificity in terms of which phages infect which bacteria. In most cases, the bacteriophage eventually kills the bacterial cell after propagating inside. Once it breaks open the cell and disperses out into the environment, it searches for new living cells to infect.
What bacteriophage products are available?
Most currently available bacteriophage products are for Listeria monocytogenes, Salmonella, and E. coli. These products are sold under the names ListShield™, Listex P-100™, EcoShield™, SalmoFresh™, and SalmonelexTM. All of these products have been approved for use in food by the Federal Drug Administration in the United States as well as other food safety agencies in other countries.
How do bacteriophage products kill potential pathogens in food?
Typically, you receive a jar with a liquid slurry that contains the bacteriophage. The jar of virus particles would have been produced by growing it with bacterial host and then the bacterial components are separated from the viral components. Because these are patented products, it’s hard to know exactly how they are produced, but it is likely that a non-pathogenic bacterium is used as the host. The slurry is then applied to the food where it will infect any viable cells of pathogens that it is designed to target.
How are these products applied to foods?
Manufacturers typically recommend spraying the products on the surfaces of foods to prevent post-production contamination. There are lots of photos showing how these products can be sprayed on everything from hot dogs to washed rind cheeses.
Are there potential health impacts of consuming these viruses?
Given that bacteriophages are only going to colonize bacteria, it is impossible for the virus to colonize human cells. There is a possibility that these viruses once consumed could kill some of the beneficial bacteria in your gut microbiome, but the manufacturers suggest that they are specific to only the pathogens they are intended to target.
Are these viruses currently being used in foods I consume?
This is a tough question to answer. Because these viruses are considered a processing aid (at least in the United States) they don’t need to be listed as an ingredient on the food you consume. So presumably, many food production companies could be using this product and you wouldn’t know.
Do these products completely eliminate pathogens from foods?
Not necessarily. As the manuals that support these products describe and as research papers demonstrate, these products will reduce the numbers of pathogens, they don’t completely eliminate them. Some pathogens could be buried deep in food materials or under layers of other microbial cells. If there are barriers that prevent the bacteriophage from infecting all cells, then the pathogen will persist.
Have these products been shown to work in fermented foods?
There have been a series of peer-reviewed papers exploring the use of bacteriophages in a wide range of foods. The main fermented food where bacteriophage biocontrols have been studied is cheese. In one study of white mold (Camembert-type) and washed-rind (Limburger-type) cheeses, scientists found that Listeria reduction was dependent on how often the product was applied as well as the initial level of contamination. Another study in Brazil came to very similar conclusions when studying two fresh cheeses. Research on the effectiveness of these products in other fermented foods where Listeria can be a problem, such as salami, has not been published.
Is this a magic bullet for food safety or are there potential limitations of bacteriophages?
As noted above, these products don’t completely eliminate all pathogens in a food product. The best way to avoid contamination of food with pathogens is to keep it out of the production and post-production process. These products may be helpful as just one small component of an integrated pathogen management approach where other safety precautions reduce the risk of pathogen contamination.
There are also some concerns with pathogens developing resistance to these viruses. Just as all humans don’t respond in exactly the same way to the flu virus, the same species of bacteria don’t all respond in the same way to bacteriophages. Some bacteria could be naturally resistant to phages and therefore these commercial products may not work well. In fact, in this paper where Listex was applied to cheese, the scientists observed that one Listeria strain survived the Listex bacteriophage application, while other strains remained susceptible during the 22 day experiment. This development of resistance is a potential pitfall in terms of the reliability of these products.
Post written by Benjamin Wolfe. Header photo by Dr Graham Beards
For more information on using bacteriophages to control pathogens, see the following papers:
S. Hagens and M.J. Loessner. 2014. Phages of Listeria offer novel tools for diagnostics and biocontrol. Frontiers in Microbiology 5:159
R.P. Pulido et al. 2015. Application of bacteriophages in post-harvest control of human pathogenic and food spoiling bacteria. Critical Reviews in Biotechnology. In press.