Some bacteriophages have developed tropism-switching genetic cassettes, that allow them to modify their fibers proteins, and thus to bind to a broader range of hosts.
Different mechanisms can be used:
- The host range variation system found in phages such as Mu and P1 is a genetic system that allows some bacterial viruses to switch between two types of fibers by inversion of the DNA segment encoding the fibers. The change in fibers specificity is performed through site specific inversion of a genome segment catalyzed by a virally encoded invertase. This genome segment codes for two alternate sets of tail fibers, of which only the "well oriented" one is expressed .
- Bordetella bacteriophages generate diversity in a gene that specifies host tropism produced through a genetic element that combines the basic retroelement life cycle of transcription, reverse transcription and integration with site-directed, adenine-specific mutagenesis .