Caudoviricetes

Upon encountering a host bacterium, the tail section of the virion binds to receptors on the cell surface and delivers the DNA into the cell by use of an injectisome-like mechanism (an injectisome is a nanomachine that evolved for the delivery of proteins by type III secretion). The tail section of the virus punches a hole through the bacterial cell wall and plasma membrane and the genome passes down the tail into the cell. Once inside the genes are expressed from transcripts made by the host machinery, using host ribosomes. Typically, the genome is replicated by use of concatemers, in which overlapping segments of DNA are made, and then put together to form the whole genome.^[2]

Viral capsid proteins come together to form a precursor prohead, into which the genome enters. Once this has occurred, the prohead undergoes maturation by cleavage of capsid subunits to form an icosahedral phage head with 5-fold symmetry. After the head maturation, the tail is joined in one of two ways: Either the tail is constructed separately, and joined with the connector, or the tail is constructed directly onto the phage head. The tails consist of helix based proteins with 6-fold symmetry. After maturation of virus particles, the cell is lysed by lysins, holins, or a combination of the two.^[2]

For most of virological history, Caudoviricetes which was known as the order Caudovirales, had lower taxa defined via morphology and contractile ability of their "tails". The Myoviridae had long tails that were contractile; the Podoviridae had short noncontractile tails; and the Siphoviridae had long noncontractile tails.^[4] Siphoviridae constitute the majority of the known tailed viruses.^[2][5]

Bradley referred to what was known as the Myoviridae as type A, Siphoviridae as type B, and the Podoviridae as type C. He also divided his groups on the basis of head morphology: Within group A, A1 have small isometric heads; A2 have prolate heads; and A3 have elongated heads. Within groups B and C, numbers were similarly assigned: B1 and C1 have small isometric heads; B2 and C2 have prolate heads; and B3 and C3 have elongated heads.^{[citation needed]}

Because the "families" Myoviridae, Podoviridae and Siphoviridae were abolished for being polyphyletic, there are now many free-floating families, subfamilies, and genera in the class without any preceding taxa before Caudoviricetes. There are currently 7 orders, 63 families, 109 subfamilies, 1360 genera, and 4079 species in the class. This article lists all official and proposed taxa of Caudoviricetes. (Note: quotation marks mean that the taxon is proposed and has not yet been ratified by the ICTV.)

• Order Crassvirales
  • Family Crevaviridae
    • Subfamily Coarsevirinae
    • Subfamily Doltivirinae
  • Family Intestiviridae
    • Subfamily Churivirinae
    • Subfamily Crudevirinae
    • Subfamily Obtuvirinae
  • Family Steigviridae
    • Subfamily Asinivirinae
  • Family Suoliviridae
    • SubfamilyBearivirinae
    • SubfamilyBoorivirinae
    • SubfamilyLoutivirinae
    • SubfamilyOafivirinae
    • Subfamily Uncouvirinae
• Order Juravirales
• Order Kirjokansivirales
• Order Magrovirales
• Order Methanobavirales
• Order Nakonvirales
• Order Thumleimavirales

There are many unassigned taxa in Caudoviricetes as of ICTV (2022).^[3]

• Family Suoliviridae
  • Other assigned subfamilies
  • Genus Dechshavirus

Crassvirales infects bacteria and is named after the computer program crAss, which was used to identify the first member of the Crassvirales order.^[6][7]

The rest (including the proposed orders) infect archaea. Kirjokansivirales, Thumleimavirales, and Nakonvirales are named after mythological objects or deities; Methanobavirales and Magrovirales are named after archaic synonyms of the archaea they infect.

Bacteriophages occur in over 1100 bacterial or archaeal genera.^[3] Over 6300 bacteriophages have been examined in the electron microscope since 1959. Of these, more than 96 percent have tails. Of the tailed phages, about 57 percent have long, noncontractile tails ("Siphoviridae"). Tailed phages appear to be monophyletic and are the oldest known virus group.^[5][8]

• WO virus