Summary
Bacteriophages (or phages) are viruses which only infect bacteria. They replicate by infecting their bacterial hosts and either killing them and releasing many progeny phage or by budding from the cells while the host remains alive. In some cases the phage can remain dormant in the host bacterium and replicate with it until the time is right for the infection cycle to complete and progeny phage to be released. A diagram of an example phage is shown to the left. The diagram shows a model of one of the most well known and well characterised phages, bacteriophage lambda. This was used as a model system for many early experiments in molecular biology and for many of the initial bacteriophage vaccine trials.

Background

Bacteriophages were independently discovered by Frederick Twort in 1915 and Felix D’Herelle in 1917, with the latter coining the term bacteriophages, which literally translates as “bacteria eaters”. The ability of phages to kill bacteria led them to the proposal that they could be used as potential antibacterial agents for the treatment of disease. However, the advent of antibiotics led to them being largely forgotten. Recently, however, there has been a resurgence of interest in phages to treat antibiotic resistant strains as well as their use as vaccine delivery vehicles, in bacterial typing and nanotechnology.

Abundance.
Phages are the most numerous form of life on the planet, existing wherever their bacterial hosts are found. It has recently been estimated that there are 109 (1 000 000 000) tailed bacteriophages per ml (millilitre or thousandth of a litre) in some fresh water sources and 1030 (1 followed by 30 zeroes) globally. If the masses of all the phages found on Earth were added up, they would weigh 1000 times the mass of all the elephants combined. Phages are found in all environments where bacteria are present.

Physical properties

Bacteriophages are usually between 20 and 200 nanometres (billionths of a metre) and consist of a protein coat surrounding the genetic material, which can be either DNA or RNA. Their genome size is from around 20 to 500 thousand bases, compared to 3 billion for the human genome or 4.6 million for an E. coli bacterium.

Uses
In the early days of phage research, the phage's ability to kill bacteria led them to be used in phage therapy, as a means to treat bacterial infections. Subsequently, their ease of production and relative amenability to experimentation, led them to be used as models for many of the early experiments that determined how organisms replicate and heralded in the era of genomics with the many health and lifestyle benefits it has brought. More recently there has been a resurgence of interest in phage with their potential uses in fields as diverse as nanotechnology, vaccine delivery, treatment of antibiotic resistant infections, detection of pathogens and development of new biopharmaceuticals. Recently, a bacteriophage-based spray to prevent listeria contamination of meat has been approved by the FDA.

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