Bacteriophage, also known as phage, are viruses that infect bacteria and archaea. Phage display a vast diversity of shapes and sizes, with some virions being as large as 700 nm in length. Phage were first discovered in the early 20th century by Frederick Twort, who observed their ability to lyse bacterial cells. However, it was not until 1915 that Felix d’Herelle coined the term “bacteriophage” after observing their effect on the Shigella flexneri bacterium.
Phage infection results in the lysis of the host cell, releasing progeny phage into the environment to infect other cells. During lytic infection, phage DNA is injected into the cytoplasm of the host cell where it is replicated using host enzymes. The viral genome is then packaged into new virions which bud from the host cell membrane. In temperate phages, however, instead of causing lysis upon infection, these viruses can enter a latent state known as lysogeny. In this state, viral DNA is integrated into the bacterial chromosome where it can be transcribed and replicated along with the rest of the genome. Lysogenic infections are often characterized by a quiet phase where no obvious signs of infection are present followed by an explosive phase where massive numbers of virions are produced and released (known as induction).
While most research on bacteriophage has focused on their role in disease and biocontrol applications, these viruses have also been exploited for various industrial and environmental uses such as wastewater treatment and food safety initiatives. Bacteriophage therapy is an emerging area of medicine that holds promise for treating drug-resistant bacterial infections which are becoming increasingly difficult to manage with traditional antibiotics alone. As our understanding of these unique viruses continues to grow, so too does our ability to harness them for a variety of purposes both inside and outside the laboratory setting.