Bacterial viruses: an introduction

Studies of bacterial viruses or bacteriophage, from the Greek meaning "bacteria eater", played an critical role in defining the molecular principals of life.

"Phage" are simple (compared to bacteria) and the speed at which they reproduce makes it possible to do experiments quickly.

In this lab, you will learn the basics of working with phage and how phage were used to define the origins of mutations.   Phage are classified into three generic types based on their "lifestyles": these are virulent, temperate and chronic. 

Temperate phage can produce one of two effects.  Like virulent, they can also induce host cell lysis.

Virulent phage, such as the T-even phage that we will be using, kill their host in a catastrophic lysis that releases many virions (infectious viral particles).


Alternatively, the virus can integrate its genome into the genome of the host.

In this state the bacterial host is said to harbor a prophage.

A prophage can actually benefit the host, protecting it from infection by other bacterial viruses.

If the host cell is threatened, however, the prophage can becomes lysogenic.

It emerges from the host genome, replicates and escapes, killing the host through lysis.


A prophage's "decision" on whether to turn lytic or remain lysogenic is a classic example of a molecular switch

Chronic phage replicate slowly and are released without killing the host cell. Again, they can protect the host from infection by other phage!


Phage differ greatly from one another and range from simple icosohedral and rod-shaped forms to extremely complex macromolecular machines.

The largest viruses discovered to date, the miniviruses, which grows in amoeba, have enough DNA to encode ~900 proteins. 

The most complex of the bacteriophages, such as T4, are 300 nm in length, contain 160,000 or more base pairs of DNA, and encode ~300 proteins.


T4 looks very much like what it is, a molecular hypodermic syringes. 

This syringe is composed of proteins; its function is to recognize a suitable host and to then inject that cell with the phage's nucleic acid.

Phage and their host bacteria co-evolved and their interactions are critical in the overall ecological dynamics of the earth.

Moreover, phage can transfer genetic material from one bacterium to another, a process known as transduction.

Transduction can play a roll similar to sex in higher organisms

Studies on how phage are constructed were the first to define the basic principles underlying protein folding and the assembly of macromolecular machines.
  • How is a bacteriophage like an allosteric enzyme?  What is the role of the host cell in that allostery?

Use Wikipedia | revised 20-Apr-2006