The Discovery of bacteriophages

THE DISCOVERY OF BACTERIOPHAGES

Viruses that infect bacteria were discovered independently by Frederick W. Twort in England in 1915 and by Felix d’Herelle at the Pasteur Institute in Paris in 1917. Two observed that bacterial colonies sometimes dissolved and disappeared because their component cells Lysed, or burst. More interesting had an observation that this lytic effect could be transmitted from colony to colony. Even highly diluted material from a lysed colony that had been passed through a bacterial filter could lyse other bacteria. However, heating this filtrate destroyed its lytic property. From this observation, Twort cautiously suggested that the lytic agent might be a filterable infectious agent.

When d’Herelle discovered his phenomenon (hence the term Twort-d’herelle phenomenon), he coined the word Bacteriophage, which means “bacteria eater”. He concluded that the filterable agent was an invisible entity- a virus- that was parasitic toward bacteria.

MORPHOLOGY AND CHEMICAL COMPOSITION OF BACTERIOPHAGES

Microbiologists have been able to separate bacteriophage structures and determine their chemical content. Like other viruses, all phages have nucleic acid core covered by the protein- containing capsid, which protects the core from nuclease and other harmful substances. The phage core usually consists of a single nucleic acid molecule, which may be single- or double- stranded linear or circular DNA, or single- stranded linear RNA. (The one knew exception is phage Φ6, which has three linear double-stranded RNA molecules whose base sequences differ from one another). The threadlike appearance of a DNA molecule extruded from a T2 phage.

There are three basic morphological forms of bacteriophages: icosahedral head with no tail, icosahedral head with a tail, and filamentous. As indicated earlier, the icosahedral heads of some phages have lengths longer than their widths. In a filamentous phage, the nucleic acid is in an extended helical form along the length of the protein coat. The phage tail may be very short or up to 4 times the length of the heads, and it may be flexible or rigid. A complex base plate may also be present on the tail; it typically has from one to six tail fibers.

CLASSIFICATION AND NOMENCLATURE OF BACTERIOPHAGES

The common names of bacteriophages do not follow particular guidelines. They are simply designations or code symbols assigned by investigators. Although serving the practical need of laboratories, this is a haphazard way of named a group of infectious entities.

Consequently, the International committee for Taxonomy of viruses (ICTV) has a Bacterial Virus Subcommittee working on the classification and nomenclature of bacteriophages. The families of bacterial viruses (names ending in –viridea) . They are grouped on the basis of the clear-cut difference in morphology and chemical composition. (Virus examples are named in parentheses in the figure). It may be pointed out that family Cystoviridae includes only one virus, whereas the largest family, Siphoviridae, has over 1200 members. More than 95 percent of the known viruses infecting eubacteria belong to one of the three families of long-tailed phages . The morphology of most of the families of bacteriophages is depicted.

System of Classification Based on Differences in Transcriptional Processes

In 1971, David Baltimore (a Nobel laureate for his work on tumor viruses) proposed a classification of viruses, with a unifying concept based on how the viral genome is replicated and expressed. All viruses were placed into one of the six classes according to the particular pathway of mRNA synthesis involved. .A central role is assigned to mRNA since protein synthesis takes place by the same mechanism in all cells. Although this scheme groups together viruses with similar replicative steps, it also groups very different virions together in the same class (for example, bacteriophages and animal viruses). It also does not take into account other properties of viruses. Such a scheme finds fever with molecular biologists who study viruses. But biology-minded virologists prefer a more general approach modeled after Linnaeus’ classification scheme, using nomenclature for families, genera, and species, the Baltimore scheme of classification of viruses based on differences in transcriptional processes.

Bacteriophages of Escherichia coli

The most extensively studied group by bacteriophages are the coliphages, a name that refers to phages that infect the bacterium Escherichia coli. One group of coliphages that infect E. Coli is designated T1 to T7(“T” refers to “type”). All the viruses in this group are composed almost exclusively of DNA and protein in approximately equal amounts. Except for T3 and T7, all have tadpole shapes, with polyhedral heads and long tails; the tails of T3 and T7 are very short . The T Phages range from about 65 to 200 nm in length and 50 to 80 nm in width. The looped molecule of double-stranded DNA (about 50 um long, or about 1000 times as long as the Phages itself) is tightly in the protein head.

Other coliphages have morphology and chemical composition very different from those of the T phages. For example, the f2 phages (“f” for filamentous) is much smaller than the T phages, has a single-stranded linear molecule of RNA rather than DNA, and lacks a tail.

There are also coliphages that process single-stranded DNA. Morphologically they can be either icosahedral or filamentous. An icosahedral phage with circular single-stranded DNA is ΦX174.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.