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Virus classification
Group: Group I (dsDNA)
Order: Caudovirales
Family: Siphoviridae

Siphoviridae are a family of double-stranded DNA viruses infecting only bacteria. These viruses may also be referred to as bacteriophages. The characteristic structural features of this family are a nonenveloped head and noncontractile tail.


Electron micrographs of siphovirus from P. acnes. Phages were negatively stained and subjected to transmission electron microscopy. The phages have a head of approximately 55 nm in diameter, loaded with genetic material. Their tails have a size of 150 × 10 nm and are flexible and non-contractile. In the lower micrograph, PAD25 is adhering to bacterial cell debris, and two phages have lost their heads. All phages were classified as Siphoviruses based on their morphology.[1]

Virions of viruses in this family have an icosahedral capsid (morphotype B1) or a prolate capsid (morphotype B2). These capsids have no envelope and are generally about 60 nanometers in diameter. Members of this family are also characterized by their filamentous, cross-banded, noncontractile tails, usually with short terminal and subterminal fibers.

The genome of Siphoviridae phages is double stranded and linear. It is typically about 50 kilobases in length and contains about 70 genes. The guanine/cytosine content is usually around 52%.


The organisation of genes in the genera L5likevirus, Lambdalikevirus, Phic3unalikevirus, and Psimunalikevirus and the proposed genera Sfi2likevirus, Sfi1likevirus, Sk1likevirus, and Tm4likevirus suggests that these may form a lamba supergroup (or subfamily).[2]

The c2-like viruses have oblate heads.

The following viruses have been classified into genera:[3][4][2]

Proposed Genera[edit]

The following genera have been proposed but are not currently ratified by the International Committee on Taxonomy of Viruses:[5][6]


In addition to the above viruses, many members of Siphoviridae have been grouped into an unclassified group with no genus assignment.[7] This group includes numerous phages known to infect Lactobacillus, Mycobacterium, Streptococcus, and other groups of bacteria.


  1. ^ Lood R, Mörgelin M, Holmberg A, Rasmussen M, Collin M (2008). "Inducible Siphoviruses in superficial and deep tissue isolates of Propionibacterium acnes". BMC Microbiol. 8: 139. doi:10.1186/1471-2180-8-139. PMC 2533672. PMID 18702830. 
  2. ^ a b Brüssow H, Desiere F (2001). "Comparative phage genomics and the evolution of Siphoviridae: insights from dairy phages". Mol Microbiol 39 (2): 213–22. doi:10.1046/j.1365-2958.2001.02228.x. 
  3. ^ NCBI Taxonomy (Siphoviridae)
  4. ^ International Committee on Taxonomy of Viruses
  5. ^ "The Dilemma of Phage Taxonomy Illustrated by Comparative Genomics of Sfi21-Like Siphoviridae in Lactic Acid Bacteria". Journal of Bacteriology. September 2014. doi:10.1128/JB.184.21.6026-6036.2002. 
  6. ^ "Three proposed new bacteriophage genera of staphylococcal phages: “3alikevirus”, “77likevirus” and “Phietalikevirus”". Archives of Virology. 11 Sep 2013. doi:10.1007/s00705-013-1833-1. 
  7. ^ "unclassified Siphoviridae". NCBI Taxonomy. 

External links[edit]