Viruses infecting bacteria are known as bacteriophages (L. bacterium + Gk. phagein, to eat) or phages or bacterial viruses. They were discovered by Twort in 1915. They are widely distributed in nature, and are especially abundant in enteric bacteria which inhabit the intestine of man and other animals. There are several varieties of phages. Each variety is specific to a particular species or strain of bacteria.
 Bacteriophages structures of (a) T4, (b) lambda and (c) M13.
Bacteriophages have tadpole like structure, i.e., with head and tail. Inside the head is present nucleic acid generally DNA. Their genomes can be made from either DNA or RNA. They can be linear or circular, single stranded or double stranded.
Types of Bacteriophages with examples
Bacteriophages are of four types: single stranded DNA Phages, single stranded RNA Phages , double stranded DNA Phages and double stranded RNA Phages.
1. ssDNA (single stranded DNA) Phages
      a) Icosahedral (isometric) phages
          eg: G viruses (G4, G6, G13,G14), φX 174
      b) Helical (Filamentous) phages:
        eg: M13, fd
2. ds DNA (double stranded DNA) phages
      eg: T even (T2, T4, T6) and T odd phages (T1, T3)
3. ss RNA (single stranded RNA) phages
     eg:M12 and MS2 phages
4. ds RNA (double stranded RNA)
     eg: φG
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