Siphoviridae
![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.[4]](/Images/godic/202502/11/lossy-page1-220px-Siphovirus.tiff3808.jpg")
Siphoviridae is a family of double-stranded DNA viruses in the order Caudovirales. Bacteria and archeabacteria serve as natural hosts. There are currently 313 species in this family, divided among 47 genera. The characteristic structural features of this family are a nonenveloped head and noncontractile tail.