Bacteriophages

Bacteriophages, also known as phages, are viruses that infect and replicate only in bacterial cells. They are ubiquitous in the environment and recognized as the earth's most abundant biological agent. They are extremely diverse in size, morphology, and genomic organization. However, all consist of a nucleic acid genome encased in a shell of phage-encoded capsid proteins, which protect the genetic material and mediate its delivery into the next host cell. Electron microscopy has allowed the detailed visualization of hundreds of phage types, some of which appear to have "heads," "legs," and "tails." Despite this appearance, phages are non-motile and depend upon Brownian motion to reach their targets. Like all viruses, bacteriophages are very species-specific about their hosts and usually only infect a single bacterial species or even specific strains within a species. Once a bacteriophage attaches to a susceptible host, it pursues 1 of 2 replication strategies: lytic or lysogenic. During a lytic replication cycle, a phage attaches to a susceptible host bacterium, introduces its genome into the host cell cytoplasm, and utilizes the host's ribosomes to manufacture its proteins. The host cell resources are rapidly converted to viral genomes and capsid proteins, which assemble into multiple copies of the original phage. As the host cell dies, it is either actively or passively lysed, releasing the new bacteriophage to infect another host cell. In the lysogenic replication cycle, the phage also attaches to a susceptible host bacterium and introduces its genome into the host cell cytoplasm. However, the phage genome is instead integrated into the bacterial cell chromosome or maintained as an episomal element where, in both cases, it is replicated and passed on to daughter bacterial cells without killing them. Integrated phage genomes are termed prophages; the bacteria containing them are termed lysogens. Prophages can convert to a lytic replication cycle and kill their host, most often in response to changing environmental conditions.