Bacteriophages, or phages for short, are viruses that infect bacteria and hijack bacterial cellular machinery to reproduce themselves. Phages are extremely abundant entities, and could be responsible for up to 20-40% of bacterial mortality daily :ref{doi=10.1038/s41586-019-1894-8}. Therefore, phage infection constitutes a very strong evolutionary pressure for bacteria.
Bacteria and their phages have co-existed for billions of years. The pressure of phage infection is thought to be a major driver of bacterial evolution and has favored the development of a diversity of anti-phage weapons. These weapons, namely anti-phage defense systems can be defined as single genes or groups of genes that partially or fully inhibit phage infection. For reviews on anti-phage systems, see : :ref{doi=10.1038/s41586-019-1894-8, 10.1146/annurev-micro-020722-013730, 10.1016/j.mib.2005.06.006, 10.1038/s41579-023-00934-x}.
In response to this evolutionary pressure, bacteria have developed an arsenal of anti-phage defense systems. The term "defense system" here designates either a single gene or a set of genes, which expression provides the bacteria with some level of resistance against phage infection.
## A brief history of anti-phage systems
## History
The first discovered anti-phage system, a Restriction-Modification (RM) system, was described in the early 1950s ref{doi=10.1128/jb.64.4.557-569.1952, 10.1128/jb.65.2.113-121.1953}. In the following decades, a handful of other systems were discovered :ref{doi=10.1016/j.mib.2005.06.006}. In 2007, CRISPR-Cas systems were discovered to be anti-phage systems :ref{doi=10.1126/science.1138140}. As CRISPR-Cas systems and RM systems are extremely prevalent in bacteria, it was thought for some years that the antiviral immune system of bacteria had been mostly elucidated.
The first anti-phage defense system was discovered in the early 1950s by two separate teams of researchers :ref{doi=10.1128/jb.64.4.557-569.1952}, :ref{doi=10.1128/jb.65.2.113-121.1953}. Luria and Human reported a mysterious phenomenon, where one phage was only capable of infecting a specific bacterial strain once. The progeny phages produced by this first round of infection had lost their ability to infect the same strain again, yet remained able to infect other bacterial strains. For them, thiscould only mean that "the genotype of the host in which a virus reproduces affects the phenotype of the new virus" :ref{doi=10.1128/jb.64.4.557-569.1952}. A similar phenomenon was shortly after described by Bertani and Wiegle.
Following these two major breakthroughs, knowledge of anti-phage systems remained scarce for some years. Yet, in 2011, it was revealed that anti-phage systems tend to colocalize on the bacterial genome in defense-islands :ref{doi=10.1128/JB.05535-11}. This led to a guilt-by-association hypothesis: if a gene or a set of genes is frequently found in bacterial genomes in close proximity to known defense systems, such as RM or CRISPR-Cas systems, then it might constitute a new defense system. This hypothesis was tested systematically in a landarmark study in 2018 :ref{doi=10.1126/science.aar4120} leading to the discovery of 10 novel anti-phage systems. This started the uncovering of an impressive diversity of defense systems in a very short amount of time :ref{10.1038/s41579-023-00934-x}.
Their work was in fact the first report of what would later be named Restriction-Modification ([RM](/defense-systems/rm)) system, which is considered to be the first anti-phage defense system discovered.
To date over 150 types of defense systems have been described, unveiling an unsuspected diversity of molecular mechanisms. The antiviral immune systems of bacteria therefore appear much more complex than previously envisioned, and new discoveries do not seem to be slowing down.
The sighting of a second defense system occured more than 40 years later, in the late 1980s, when several teams around the world observed arrays containing short, palindromic DNA repeats clustered together on the bacterial genome :ref{doi=10.1038/nmicrobiol.2017.92}. Yet, the biological function of these repeats was only elucidated in 2007, when a team of researchers demonstrated that these repeats were part of a new anti-phage defense systems :ref{doi=10.1126/science.1138140}, known as [CRISPR-Cas system](https://en.wikipedia.org/wiki/CRISPR).
## Introducing the defense finder wiki
Following these two major breakthroughs, knowledge of anti-phage systems remained scarce for some years. Yet, in 2011, it was revealed that anti-phage systems tend to colocalize on the bacterial genome in defense-islands :ref{doi=10.1128/JB.05535-11}. This led to a guilt-by-association hypothesis: if a gene or a set of genes is frequently found in bacterial genomes in close proximity to known defense systems, such as RM or CRISPR-Cas systems, then it might constitute a new defense system. This hypothesis was tested systematically in a landarmark study in 2018 :ref{doi=10.1126/science.aar4120} leading to the discovery of 10 novel anti-phage systems. This started the uncovering of an impressive diversity of defense systems in a very short amount of time :ref{10.1038/s41579-023-00934-x }.
The fast pace of discoveries in the field can be intimidating to newcomers and can make it difficult for all to keep track of new discoveries. For this reason, we decided to implement a collaborative knowledge base for the community. This wiki is divided in two sections:
1. A “general concepts” section, introducing key notions and ideas to understand anti-phage defense
2. A section introducing succinctly each of the defense systems currently known.
## List of known defense systems
This wiki is only a first version, and is intended to evolve based on the ideas and needs of the people using it. Whether it is to suggest new pages or to edit existing ones, all contributions are more than welcomed: please do not hesitate to contact us to participate!
To date, more than 150 anti-phage defense systems have been described. An exhaustive list of the systems with experimentally validated anti-phage activity can be found [here](/defense-systems).
