The thesis deals with all currently known bacterial defense mechanisms against bacteriophages. The theme is divided into sections that correspond with those of a bacteriophage life cycle. The first step is to prevent the binding of a bacteriophage to the bacterial cell membrane. Cells achieve this by blocking access to their receptor. Access may be limited by the production of an extracellular matrix, masking elements or competitive inhibitors. In the second step, the cell is trying to prevent entry of foreign DNA after a successful phage binding to the receptor. Systems that perform this task are called Sie. Failure of Sie systems leads to the activation of R-M systems, which cut the foreign nucleic acid before it manages to integrate into the DNA of the host cell. Mechanism similar to the R-M system is the CRISPR-Cas system, which, unlike other defensive mechanisms functions as adaptive immunity. If all these systems fail, it leads to the cell resorting to suicide as an altruistic defense for other cells in the population, enabled by Abi and TA systems. Viral assembly interference is not bacteria’s own mechanism, but depends on the presence of a parasite in the form of autonomously replicating DNA. In the process of assembly viral DNA is replaced by the parasite’s. Thereby preventing viral replication, but not preventing host cell lysis. Finally, it describes the latest defense mechanism called BREX.
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