Center for Phage Research and Phage Therapy, Regensburg, Germany

Bacteriophages (“bacteria eaters”) or phages are viruses that infect bacteria in a highly specific manner and are ubiquitous in the biosphere. Based on estimates from seawater studies, there are approximately 1031 bacteriophages in the biosphere, making them the most abundant replicating entities on our planet.

Phages differ greatly in terms of morphology, genome type and size and life cycle. The virions of phages form spherical and filamentous structures, some of which have a complicated tail. The capsids of phages can consist only of proteins or be covered with a lipid membrane doublelayer. The genome of phages can be linear or circular and consist of single- or double-stranded DNA or RNA. The smallest genomes are only a few kilobases (kb) in size, while the largest known genomes have a size of 735 kb. Bacteriophages replicate either lytically or lysogenically, i.e. the bacterial cell integrity is destroyed immediately after replication of the phage or the phages replicate lysogenically and become temperent, whereby their viral genome integrates into the host genome.

In general, phages cannot actively move, but only reach their host-receptor by diffusion. The mechanisms of infection are only partially understood. Myovirus bacteriophages, for example, form a kind of molecular syringe that injects its genetic material into the cell. After infection, phages use the host's protein synthesis machinery to synthesize new descendants.