Bacteriophages

Bacteriophages - eaters of bacteria

Since their discovery in the early 20th century, bacteriophages have been one of the most important model systems in biology. In the 1930s and subsequent decades, pioneering virologists utilized these viruses 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 which infect humans, but are much more difficult to propagate and study. Phages are still a paradigm for many areas of biology, especially gene expression.

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. They are also of industrial and economic importance. e.g:

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

This video shows plaques (clear areas) produced in a lawn of bacterial cells by the replication of phages and consequent death (lysis) of the bacterial cells. The dark spots on the agar plate are colonies of bacteria which are resistant to the phages:

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, a permanently infected state. 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.

The "single burst experiment" (Ellis E.L, Delbruck M. 'The growth of bacteriophage' J.Gen.Physiol. 22: 365-384, 1939) revealed the nature of virus replication. You can perform this experiment for yourself online!

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by Mark Ptashne
A beautiful in-depth examination gives an appreciation for molecular biology at a comfortably sophisticated level. This extended edition includes higher organisms, focusing on how an organism uses its genes to direct its growth. New chapters assess how well the lambda model can explain gene control in eukaryotes, and the elaborations necessary to explain more complex systems.
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Microbiology
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