section_path: Infection in other species/Bacterial viruses
section_name: Bacterial viruses
paragraph_text: Bacterial viruses
  
  Bacteriophages are a common and diverse group of viruses and are the most abundant biological entity in aquatic environments—there are up to ten times more of these viruses in the oceans than there are bacteria, reaching levels of 250,000,000 bacteriophages per millilitre of seawater. These viruses infect specific bacteria by binding to surface receptor molecules and then entering the cell. Within a short amount of time, in some cases, just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins, which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane, and, in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released.
  
  The major way bacteria defend themselves from bacteriophages is by producing enzymes that destroy foreign DNA. These enzymes, called restriction endonucleases, cut up the viral DNA that bacteriophages inject into bacterial cells. Bacteria also contain a system that uses CRISPR sequences to retain fragments of the genomes of viruses that the bacteria have come into contact with previously, which allows them to block the virus's replication through a form of RNA interference. This genetic system provides bacteria with acquired immunity to infection.
  
  Some bacteriophages are called " temperate " because they cause latent infections and do not immediately destroy their host cells. Instead, their DNA is incorporated with the host cell's as a prophage. These latent infections become productive when the prophage DNA is activated by stimuli such as changes in the environment. The intestines of animals, including humans, contain temperate bacteriophages, which are activated by various stimuli including changes in diet and antibiotics. Although first observed in bacteriophages, many other viruses are known to form proviruses including HIV.