Listeria’s worst enemy
Bacteriophage are viruses that attack bacteria. These viruses have among other things been used to fight infection in human beings. Now scientists are looking at ways to use bacteriophage to fight Listeria in food production.
A significant amount of food poisoning cases in the world are caused by viruses. Because of this, we human beings are innately skeptical of using viruses in foods. Askild Holck reassures us that bacteriophage, also known simply as phage, are in no way harmful to human beings. Bacteriophage naturally occur in food, and fortunately for us, some are specially adapted to attack Listeria Monocytogenes.
"Phage attach themselves to a bacterial host and transmit their own DNA into the bacterium. This starts a process culminating in the bacterium producing more bacteriophag. When the bacterium contains a large amount of phage, it bursts, releasing the phage. These phage are now able to attack other bacteria. Phage are thus parasites, only reproducing when they enter a bacterium," senior scientist Askild Holck explains.
"Should a bacteriophag end up on a different type of bacteria, it would become very confused as to what it should do." Askild Holck draws a parallel: "It would be as though a burglar was trying to break into a bank, only to discover that he had entered a classroom. He would be very confused," Askild Holck smiles.
Good results for ham
At Nofima Mat Askild Holck and his co-workers have tested the virus on Listeria infected ham. They tested how phage alone and phage combined with lactobacilli could kill Listeria at different temperatures. Results were very good when the virus and lactobacilli were used in combination, and looks promising with regards to bacteriophag as a hurdle against the growth of Listeria. The efficiency of bacteriophage, and bacteriophage in combination with lactobacilli will vary from product to product.
"Bacteriophage and lactobacilli in combination in boiled ham may kill 99.99% of the Listeria. We will keep researching this. Finding robust combinations that together are able to eliminate Listeria is key," says Askild Holck.
Wanting to kill bacteria
"We’re constantly searching for ways to make food safer. To be on the safe side, we can boil the food to death, but no one wants that," says Holck. There are great number of strategies that can be employed to make food safer. Some of these are somewhat innovative, some controversial.
Technically, there are several ways in which food can be produced safely. In principle, antibiotics could be added to food, but this is scientifically unacceptable. It is also possible to irradiate food, but this is politically unacceptable.
Testing high pressure
Another method for safer food production is high pressure treatment. Holck explains that this is done by exposing the food to high pressure (6000 atm). This forces water into the bacterium, causing the bacterium to burst when the pressure is dropped and the water expands.
"This method works excellently for a number of foods, such as liquid products and cured meats. Other products are not well suited for this method. Soft cheeses turn into sallow porridge and smoked salmon changes color," says Askild Holck.
Then there is the use of viruses, which may also give direct help, as this anecdote shows:
The use of bacteriophage is not new. Phage have been used for more than 60 years. A story from Russia relates that two woodsmen found a warm container in the forest. They took it back to their campsite for heat, with dire consequences. The container was filled with radioactive material, and the men developed large open sores. The doctor who treated the woodsmen sprayed bacteriophage into the open, infected wounds. The bacteriophage virus worked as an antibiotic, and killed the bacteria in the wounds. Antibiotics have replaced bacteriophage in medicine now, but they may get their comeback in the battle against Listeria.