Merge branch 'aherrero-main-patch-30467' into 'main'

Update 7.mge-defense-systems.md

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content/2.general-concepts/7.mge-defense-systems.md

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 title: Defense Systems and MGE
-layout: article-no-toc
+contributors: 
+    - Marian Dominguez-Mirazo
+layout: article
 ---
 
-Defense systems help bacteria against mobile genetic elements (MGE), such as phage or plasmids.
-Yet, some defense systems favor certain MGE, or some MGE carry defense systems.
+Mobile genetic elements (MGEs), such as plasmids, bacteriophages, and phage satellites, facilitate horizontal gene transfer (HGT) within microbial populations, playing a crucial role in the genetic diversity and genomic evolution of bacteria :ref{doi=10.1098/rstb.2020.0460}. These elements expedite the exchange of genetic material among bacterial cells, promoting the dissemination of advantageous traits like antibiotic resistance, virulence factors, and metabolic capabilities, allowing bacteria to adapt to dynamic environments :ref{doi=10.1098/rstb.2020.0460}. However, the presence of MGEs can impose a substantial fitness cost on the bacterial host, as in the case of lytic phage infections. To counteract parasitic genomic elements, including viruses and other MGEs, bacteria have evolved defense systems. These defense systems are often disadvantageous under low parasite pressure, leading to their occasional loss. However, as the pressure from parasites increases, these defense systems become advantageous. Consequently, defense systems in bacteria exhibit high mobility and transfer rates :ref{doi=10.1038/s41576-019-0172-9}. Interestingly, a large fraction of defense systems in bacteria are encoded by MGEs :ref{doi=10.1038/s41467-022-30269-9,10.1371/journal.pbio.3001514}. While sometimes the fitness interests of MGEs and the bacterial host are aligned, these systems are likely to be selected because they benefit the MGE encoding it rather than the host cell who :ref{doi=10.1371/journal.pbio.3001514,10.1038/s41576-019-0172-9}. This benefit may include preventing other mobile elements from infecting the same cell and competing for essential resources. The presence of defense systems can, in turn, have an effect in gene flow who :ref{doi=10.1371/journal.pbio.3001514}. 
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