smooth-rough variation

S-R variation a genetic mutation or an adaptation seen in bacteria, most often evidenced by a change in the surface of colonies from smooth (S, glossy) to rough (R, dull). The change correlates with pathogenicity, S strains being generally more virulent and R strains less so. The cells in S colonies have polysaccharide capsules and are more antigenically complete; R cells contain little or no capsule. The term may also refer to changes in other cell structures such as flagella and somatic antigens, as well as susceptibility to bacteriophage. Variations are often reversible and tend to result in mixed types on repeated subculture. See also bacterial dissociation, under dissociation.