Switch systems

Abstract

Abstract Bacteria of many species are frequently observed to exhibit profound phenotypic changes, typically affecting colonial appearance or behavior. Most often, such changes are evident in pathogenic bacteria, where genetic switches modulate expression of a variety of virulence determinants (1). Such variation is the result of a diverse range of genetic mechanisms that are manifested in what are somewhat imprecisely grouped together as either phase variation or antigenic variation. Phase variation results from quantitative changes in gene expression that may range from what are, in essence, simple on-off genetic switches, through to modulation of the volume of expression of functional gene product at either the transcriptional or translational level. In contrast, antigenic variation is evident as qualitative changes in the nature of one or more surface components. In the main, such variations are detected by sequence changes involving more complicated switches that lead to the expression of an antigenically altered protein. Antigenically variable components have been studied largely because of their importance in bacterial virulence. However, equivalent changes in surface-exposed components of non-pathogenic bacteria almost certainly occur and may have particular selective advantages in their particular environments. Moreover, qualitative changes in a variable gene product could take place without necessarily having effects on the immunological properties of the variable component concerned.