Analysis of Genetic Diversity of Salmonella typhi Isolates by Pulsed-Field Gel Electrophoresis

Choi, Youn Mi; Pai, Chik Hyun

Abstract

BACKGROUND: In many developing countries, typhoid fever remains an important cause of morbidity and mortality. In Korea, the disease is endemic with a high incidence. For an effective surveillance for this important human disease, the availability of detailed and accurate data on the molecular epidemiology of Salmonella typhi is crucial. In the present study, the pulsed-field gel electrophoresis (PFGE) technique, which has been used successfully to perform a comparative chromosomal DNA analysis, was used to assess the extend of molecular diversity among the strains of S. typhi isolated in Korea.
METHODS
Included in the study were 51 strains of S. typhi isolated at Asan Medical Center (during the period from 1989 to 1996) and 16 isolates from other hospitals in Seoul, Chunbuk, Kyungpook, Pusan, Chonbuk and Chonnam. The isolates were analyzed by PFGE following XbaI digestion of DNA. PFGE patterns were assigned arbitrary types, compared by calculating a similarity coefficient and analyzed to generate dendrogram.
RESULTS
PFGE analysis produced multiple patterns consisting of 15 to 19 fragments ranging in size 20 to 600 kb. These were arbitarily assigned 7 types, A, B, C, D, E, F, and G, and 5 and 10 subtypes within A and B, respectively. Of 54 isolates from Seoul, 9 showed the identical PFGE pattern, indicating that an outbreak of typhoid fever had occurred. Many of the identical patterns were also shared between isolates from Seoul and other areas. Similarity coefficient was between 0.606 and 1.0.
CONCLUSIONS
Although a considerable genetic diversity exists among S. typhi isolates from different areas in Korea, suggesting a sporadic occurrence of typhoid fever, the identical PFGE patterns were also found among isolates from the same geographical areas of Seoul, indicating that some outbreaks had occurred. More efforts should be directed toward the epidemiological investigation of the cases to detect outbreaks and prevent further spread of the infection. The findings that many PFGE patterns are present among the Korea isolates of S. typhi suggest that PFGE may be used effectively with a considerable degree of discriminating power for the epidemiological investigation of typhoid fever.