rRNA gene restriction patterns and biotypes of Shigella sonnei

Shigella sonnei is a major agent of diarrhoeal disease in developed as well as in developing countries. Several phenotypic methods to define strain differences have been applied to this species of Shigella including, more recently, analysis of extrachromosomal and chromosomal DNA. In this study, 432 endemic and epidemic strains isolated between 1975 and 1991 in Italy, France and Switzerland were submitted to rRNA gene restriction pattern analysis, after digestion of whole-cell DNA by Hinc II, and to concomitant biotyping. Thirteen ribotypes, H1 to H13, and five biotypes, a, d, e, f, g, were detected. Ninety-five percent of the sporadic strains were assigned to ribotypes H1 to H4, which could be subtyped, except for H4, in different biotypes. Strains from each of seven different outbreaks had indistinguishable ribotype-biotype patterns. In contrast, 65 strains, isolated in Sicily in 1980 over an extended period of apparently epidemic increase of isolations and which had previously been considered to be a single bacterial clone on the basis of resistance pattern and phage type, were found to belong to two different and scarcely related ribotypes. Ribotyping and biochemical subtyping appear to be a useful epidemiological tool in studies on the circulation and distribution of strains of S. sonnei.