Structure and Function of Bacteriophages

Abstract

Bacteriophages, or phages, are viruses with an exquisitely evolved structure to accomplish their goals. These goals are recognizing a suitable host bacterium, profiting from the host metabolism, and producing multiple progeny phages that are stable enough to survive until they find a new host bacterium to infect. Their genomes consist of single-stranded RNA, double-stranded RNA, single-stranded DNA, or double-stranded DNA, depending on phage type. They store their genome in highly symmetric protein capsids to protect it from degradation. Often these capsids are icosahedral, but helical and other shapes are also used. Tectiviridae and Corticoviridae have an internal lipid membrane, while Cystoviridae sport an outer membrane layer. Phages with tails, belonging to the Caudovirales order, are the most commonly encountered bacteriophages and have icosahedral or prolate capsids. In addition to the capsid, phages need a host cell recognition apparatus. The small icosahedral Leviviridae have a single minor capsid protein for this purpose. More complex phages dedicate multiple proteins to host cell recognition, and examples of this are the helical Inoviridae and the icosahedral Tectiviridae, Corticoviridae, and Cystoviridae. The Caudovirales have highly efficient tail protein complexes for DNA transfer. These tails are flexible (Siphoviridae), extensible (Podoviridae), or contractile (Myoviridae). Apart from elements designed for genome protection, host recognition, and genome transfer, more complicated phage particles may contain proteins for environmental sensing, binding to suitable matrices where host bacteria are likely to be encountered, and other functions.