Let us learn about Bacteriophages and Other Bacterial Viruses.
(1) The shape, structure, form and function of bacteriophages have been worked out mainly for the T series of E. coli phages. The coliphage T2 has a polyhedral head (~100 nm in length) and a tail of similar length.
(2) Most bacteriophages contain ds-DNA. However, several phages with ss-DNA and some with ss-RNA have been discovered. The RNA containing phages R17 and Qβ have one of the smallest genomes (known for phages) being: 3500-4500 nucleotides.
(3) T4 phage is an example of T even phages (e.g., 12, T4 and T6) and has a hexagonal head (~900 Å wide) and has ds-DNA covered by a protein coat. It’s hollow central core has a diameter of 80-100 Å .
(4) As early as 1915, Twort observed some degenerative changes in staphylococcal colonies isolated from calf lymph, that could be transmitted serially by application of culture extracts from the original plaques (growth).
(5) D’Herelle (1917) working on the filtrates of faeces cultures from dysentery patients, could induce transmissible lysis of a broath culture of a dysentery bacillus. He came to the conclusion that the lytic agent was a virus and gave it the name bacteriophage.
(6) The one-step growth curve of the life cycle of viral particles (e.g., T4 bacterio phage, showing lytic cycle) is also known as Ellis-Deibruk experiment. The newly formed viral progeny are measured as the number of Plaque-forming units (PFU); that represent the number of viral colonies on the plate.
(7) Still other, viruses are lysogenic viruses (e.g., bacteriophage of E. coli) called lambda (λ).
(8) Phage λ is lysogenic for E. coli K12. Such phages (temperate phages) do not destroy their host cells. It also uses its tail to release DNA into the host, that gets integrated with the bacterial chromosome (Prophage). This prophage behaves like the host chromosome and replicates synchronously with it.
This phenomenon is termed lysogeny and a bacterium that bears the prophage within its genome is called lysogenic bacterium. As certain new characters develop on the lysogenic bacterium, this is also known as lysogenic conversion or phage conversion and is brought about by the new proteins synthesized by the prophage DNA.
The resistance that a lysogenic bacterium shows towards the reinfection (to itself) by the same or related phages is called super-infection immunity.
(9) The bacteriophages can also act as carriers of genes from one bacterium to another through transduction (e.g., two types of transductions have been recognized: General and restricted transduction).
(10) There are about 60 genes on the phage genome that is contained in the Head. Some important genes are shown in the simplified genome map below (Fig. 14.2)
