Siphoviruses

Siphoviridae is a family of tailed bacteriophages within the order Caudovirales. Members are characterized by an icosahedral head containing double-stranded DNA and a long, flexible, noncontractile tail that can extend hundreds of nanometers. The tail, often equipped with long fibers and a baseplate, mediates attachment to specific bacterial hosts and influences host range. Particle morphology typically features a roughly 50–60 nm head and a tail length that can range from about 150 to several hundred nanometers.

The genomes of siphoviruses are circular or linear double-stranded DNA, typically in the 20–60 kilobase range, encoding structural proteins, replication and lysis modules, and sometimes lysogeny-related genes such as integrase and repressor proteins. Siphoviruses include both temperate and lytic phages. Lambda phage, a well-studied temperate siphovirus that infects Escherichia coli, serves as a model for lysogeny and regulatory circuits. After adsorption via tail fibers, infection may proceed through the lytic cycle, producing new virions, or integrate into the host chromosome as a prophage, with the option of reactivation under stress.

Taxonomic and ecological notes: Siphoviridae is a diverse family within Caudovirales, comprising numerous genera and a wide range of bacterial hosts. They are common in many environments and contribute to horizontal gene transfer, bacterial evolution, and ecosystem dynamics. In research and applied contexts, siphoviruses—especially lambda-like phages—have played a central role in studies of phage genetics, recombination, and host–phage interactions, and are examined for insights into phage therapy and microbial ecology.