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When the virus replicates faster than the immune system can control, it can destroy cells and harm the body, and it can even incite an over-zealous immune reaction that can cause other damage.
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
The two-domain system is a biological classification by which all organisms in the tree of life are classified into two domains, Bacteria and Archaea. [ 1 ] [ 2 ] [ 3 ] It emerged from development of knowledge of archaea diversity and challenges the widely accepted three-domain system that classifies life into Bacteria, Archaea, and Eukarya . [ 4 ]
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. [188] [189] This genetic system provides bacteria with acquired immunity to infection. [190]
A 2006 study of pathogenic bacteria in hospitals found that their ability to survive varied by the type, with some surviving for only a few days while others survived for months. [69] The lifespan of microbes in the home varies similarly. Generally bacteria and viruses require a wet environment with a humidity of over 10 percent. [70]
According to the domain system, the tree of life consists of either three domains, Archaea, Bacteria, and Eukarya, [1] or two domains, Archaea and Bacteria, with Eukarya included in Archaea. [ 3 ] [ 4 ] In the three-domain model , the first two are prokaryotes , single-celled microorganisms without a membrane-bound nucleus .
The cell wall of bacteria is also distinct from that of achaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin (produced by a fungus called Penicillium) is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan. [76]
Eubacterium is a genus of Gram-positive bacteria in the family Eubacteriaceae. These bacteria are characterised by a rigid cell wall. They may either be motile or nonmotile. If motile, they have a flagellum. A typical flagellum consists of a basal body, filament, and hook. The long filament is the organ which helps eubacteria move.