Use of molecular subtyping to document long-term persistence of Corynebacterium diphtheriae in South Dakota

Enhanced surveillance of patients with upper respiratory symptoms in a Nort hern Plains community revealed that approximately 4% of them were infected by toxigenic Corynebacterium diphtheriae of both mitis and gravis biotypes, showing that the organism is still circulating in the United States. Toxig enic C, diphtheriae was isolated from five members of four households. Four molecular subtyping methods-ribotyping, multilocus enzyme electrophoresis (MEE), random amplified polymorphic DNA (RAPD), and single-strand conformat ion polymorphism-were used to molecularly characterize these strains and co mpare them to 17 archival South Dakota strains dating back to 1973 through 1983 and to 5 isolates collected from residents of diverse regions of the U nited States. Ribotyping and RAPD clearly demonstrated the household transm ission of isolates and provided precise information on the circulation of s everal distinct strains within three households. By MEE, most recent and ar chival South Dakota strains were identified as closely related and clustere d within the newly identified ET (electrophoretic type) 215 complex. Furthe rmore, three recent South Dakota isolates and eight archival South Dakota i solates were indistinguishable by both ribotyping and RAPD, All of these mo lecular methods showed that recent South Dakota isolates and archival South Dakota isolates were more closely related to each other than to the C. dip htheriae strains isolated in other parts of the United States or worldwide, The data also supported the improbability of importation of C, diphtheriae into this area and rather strongly suggest the long-term persistence of th e organism in this region.