Abstract: Objective To observe the presence pattern of two insertion elements,IS 861 and IS 1548 in the genome of Group B streptococci(GBS);to study the relationship between the insertion elements in the genome and the phenotypes of GBS(virulence,drug resistance,and serotype);and to access the value of insertion elements in genotyping.Methods IS861 and IS1548 were studied with PCR-restriction fragment length polymorphism(RPLP),nucleic acid probe and Southern blot in 113 strains isolated in Beijing,Guangzhou and St.Petersburg.Results Of the 113 GBS strains,67(59.3%) and 13(11.5%) possessed IS861 and IS1548 respectively.There was no significant difference in positive insertion rate of the two elements among the strains isolated in the three cities.Thirteen strains with IS1548 additionally possessed IS861 and all belonged to serotype Ⅲ.Relatively larger number of insertion element copies were discovered in the genome of these strains.The GBS strains possessing insertion element(s) showed a significantly higher erythromycin resistance rate.Nine HindⅢ restriction patterns,hybridized with gene probe IS861 were found in 67 insertion element positive strains,and the copy numbers varied from 1～9;while 6 EcoRⅠrestriction patterns,hybridized with IS 1548 gene probe,were found in 13 strains with the copy number varied form 8～9.Conclusion The insertion of insertion element(s) is common in the genome of GBS.IS861 and IS1548 are closely related to the special properties of GBS serotype Ⅲ.Most of the strains of serotype Ⅲ were believed to be from a same pathogenic clone,and clinical attention should be paid to the women and neonates carrying GBS serotype III.IS861 and IS1548 may modulate the erythromycin susceptibility of GBS by affecting the expression of erythromycin resistant genes.The comparative study of different typing method suggest that the combination of genotyping by insertion elements PCR-RFLP and serotyping demonstrate a good discriminative value in epidemiological study of GBS.

Key words: streptococcus agalactiae, DNA transposable element, Genotype, Serotype, Drug resistance