(Excerpted from a course by Dr. Alan Cann, at the University of Leicester, UK)

Bacteriophages

The Importance of Bacteriophages:

Historical:

Frederick Twort (1915) and Felix d'Herelle (1917) were the first to recognize viruses which infect bacteria, which d'Herelle called bacteriophages (eaters of bacteria). In the 1930s and subsequent decades, pioneering virologists such as Luria, Delbruck and many others utilized these viruses as model systems to investigate many aspects of virology, including virus structure, genetics, replication, etc. These relatively simple agents have since been very important in the development of our understanding of all types of viruses, including those of man which are much more difficult to propagate and study. They are still a paradigm for many areas of biology, especially gene expression (See Bacteriophage Lambda) .

Environmental:

Bacteriophages, like bacteria, are very common in all natural environments and are directly related to the numbers of bacteria present. They are thus very common in soil and have shaped the evolution of bacteria.

Industrial/Economic:

Phages of Lactobacillus are a serious problem for the dairy industry.
Medical - phage typing (e.g. Staphylococcus); antibacterials - Flemming.
Recombinant DNA vectors - cloning, expression, enzymes (T4 DNA ligase)

Diversity

There are at least 12 distinct groups of bacteriophages, which are very diverse structurally and genetically; the best known ones are the common phages of E.coli :

Replication:

Essentially similar to other viruses. After attachment (or "adsorption") to a specific receptor in the bacterial cell wall, the phage genome enters the cell. The capsid proteins are usually stripped off and remain outside the cell. T-even phage tails have a contractile sheath and function "like a syringe". The "head and tail" morphology of some is unique to phages and not found among other groups of viruses and is related to their mode of cell penetration. Other "round" phages (icosahedral or lipid-coated) penetrate the bacterial cell wall by adhering to flagellae or pili and being "drawn in" to the cell.

After entry into the cell, many phage genomes are degraded and destroyed. Since phage are so prevalent in the environment, bacteria have specific mechanisms to protect themselves against infection with phage - "restriction/ modification" systems which depend on the recognition and destruction of foreign (unmodified - methylated or glucosylated) DNA.

Surviving phage genomes sequester the cellular apparatus for gene expression (transcription and translation) to various extents.

Virulent vs. Temperate Phages

• Virulent phages do not integrate their genetic material into the host cell chromosome and usually kill the host cells (lytic infection) (e.g. T-phages of E.coli ).
• Temperate phages may integrate into the host DNA, causing LYSOGENY.

The single burst experiment (Ellis and Delbruck, 1939):

The Hershey-Chase experiment (1952):

Lysogeny:

The indefinite persistence of phage genomes within bacterial cells in the absence of a productive infection but with the potential to produce progeny phage under certain circumstances was first recognised in the 1920's.
On infection with a temperate phage, vegetative replication (lysis) occurs in most cells, but a few become persistently infected due to integration of the phage genome. These lysogenized cells are immune (not resistant!) to superinfection by the same phage due to repression of transcription caused by the resident prophage (See Bacteriophage Lambda) .

Phage Genetics

On rare occasions, lysogeny permits the transfer of bacterial genes between cells by TRANSDUCTION. There are two forms:

Generalized Transduction: bacterial rather than phage DNA is packaged into a phage head. When another cell is infected, the bacterial DNA is injected and in a proportion of cases, may be incorporated into the chromosome by homologous recombination, replacing the existing genes. Frequency 10^5 - 10^8 per cell. More than one gene may be cotransduced - limit = packaging size = ~50kbp = ~1% of bacterial chromosome.

Abortive transduction occurs when the new DNA does not integrate into the chromosome - may affect the phenotype transiently but is not replicated and is eventually lost.

Specialized Transduction: Results from inaccurate excision of an integrated prophage; some phage DNA is lost and some bacterial genes are picked up and carried to the next host - therefore phage are usually defective (non-infectious) and require replication-competent helper phage to replicate, depending on which phage genes are lost.

Transposition and Insertional Mutagenesis: A few temperate phage, such as Mu, act as transposons and move from site to site in the bacterial chromosome. New insertions may result of the inactivation of bacterial genes (promoters or coding sequences).
The above mechanisms have been very important in bacterial genetics, but are now largely superseded by molecular biology, in which phage remain important as cloning vectors (M13, lambda, cosmids).