Giving bacteria a cold

I hope that title got your attention. I could equally have used "Fight fire with fire" or "Biological warfare".

Food Standards Australia New Zealand is calling for submissions on an application to use viruses to aid in the control of Listeria monocytogenes.  L. monocytogenes is a bacterium found in the environment that can infect food and may cause listeriosis.  The majority of the population is unaffected by Listeria, but it is a hazard for pregnant women, young children and the elderly, in addition to those who are immunocompromised.  Note that you don't have to have had an organ transplant to be immunocompromised - you might just have another infection stressing your immune system.

You might think that deliberately putting viruses into foods is crazy.  However, these viruses are special.  They exclusively target bacteria and are called bacteriophages.  Bacteriophages (or "phages") are extremely common; they occur at levels up to 9×10⁸ per mL in bacterial mats at the surface of the sea.  They were first identified in 1915 by Frederick Twort in England and, independently, in 1917 by Felix d'Herelle in France.

Bacteriophages are deceptively simple - essentially a piece of nucleic acid contained within a protein coat called a capsid.  They can't reproduce independently and have to have a means of infecting a living cell.  Perhaps the most famous phage is the T4 phage that infects Escherichia coli and always reminds me of the lunar lander.  (I sometimes wonder how something like this could have evolved by chance).  See image from Wikipedia below.  

 When a phage infects a bacterial cell, it injects its DNA into the cell.  The viral nucleic acid then takes over the bacterial synthetic machinery and makes copies of itself, and synthesises new phage coat and other components.  The components are then assembled into new phage particles, whereupon the bacterial cell is lysed and releases the phage.  Burst sizes may be around 100 viruses per bacterial cell.  Since these are all infective, the infection of the population proceeds rapidly, resulting in the death of the majority of the bacterial cells.  Phages cause the cheese-making industry a lot of trouble, because they kill the starter bacteria.

The application currently under consideration is for the use of a mixed bacteriophage preparation, sold commercially as LISTEX™ P100, as a processing aid.  This preparation was the first phage product to be classified as Generally Recognized as Safe (GRAS) by the FDA  and USDA.

A number of scientific papers have been published on the efficacy of the P100 preparation, showing that the phage significantly reduces the population of L. monocytogenes on foods, such as salmon fillets or surface-ripened cheeses*.

The use of bacteriophage to control pathogens, such as Listeria and Salmonella, can reduce the risk of food poisoning, though it is unlikly that it can be a total solution, as the pathogen population may not be totally destroyed.  However, when used in an integrated food safety programme, the processing aid can be a valuable tool, reducing the reliance on chemicals to inhibit the bacteria.

*  See:  doi:10.1016/j.yrtph.2005.08.005

Electron micrograph of T4 phage adsorbed to a bacterial cell.

Image by Elizabeth Kutter, Bacteriophage Ecology Group

http://www.mansfield.ohio-state.edu/~sabedon/beg_phage_images.htm

Odd Spot:  When I was researching this post, I searched on "Listeria virus".  I got many hits, predominantly in popular press and websites, where the Listeria bacteria were described as "viruses".  In modern microbiology terms, this is totally wrong.  However, "virus" is derived from the Latin word for poison and this apparently appeared in the English lexicon in 1392. "Virus" was also used in 1728 to describe an "agent that causes infectious diseases".  I'm sure that most of those press writers didn't know this, but, strictly, they were correct.