Phase variation is a method for dealing with rapidly varying environments without requiring random mutation employed by various types of bacteria, including Salmonella species. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial population. Although it has been most commonly studied in the context of immune evasion, it is observed in many other areas as well.
Phase variation is defined as the random switching of phenotype at frequencies that are much higher (sometimes >1%) than classical mutation rates. Phase variation contributes to virulence by generating heterogeneity; certain environmental or host pressures select those bacteria that express the best adapted phenotype
Salmonella use this technique to switch between different types of the protein flagellin. As a result, flagella with different structures are assembled. Once an adaptive response has been mounted against one type of flagellin, or if a previous encounter has left the adaptive immune system ready to deal with one type of flagellin, switching types renders previously high affinity antibodies, TCRs and BCRs ineffective against the flagella.
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