SUMMARY: strains causing human extra-intestinal infections may be divided into two groups, B and B according to the electrophoretic patterns of carboxylesterase B. This study compares the restriction fragment length polymorphism (RFLP) of ribosomal DNA (rDNA) for 45 B strains and 45 B strains to examine the genetic structure of B strains and to distinguish them from B strains. The isolates were chosen for diversity in their allozymes of esterases, B, A, C and I, their production of virulence factors (α-haemolysin, mannose resistant haemagglutinin and cytotoxic necrotizing factor) and certain O antigens, and their pathological and geographical origins. DNA was digested with III and HI restriction enzymes and analysed by Southern blotting. The resulting rDNA RFLP patterns of B strains were distinct from those of the B strains. Moreover, the B strains appeared to be less heterogeneous than the B strains. The B strains gave 13 ribotypes (resulting from the combination of the rDNA RFLP patterns obtained with III and HI digestions) while the B strains gave 32 ribotypes. Correspondence analysis of the data showed that several clusters of strains were identified in the B strains by particular ribotypes, certain associations of esterase B and A electrophoretic variants, O serotypes and virulence factor production. In contrast, these parameters appeared to be unrelated in the B strains, reflecting their heterogeneity. These findings, which differentiate two levels of genetic heterogeneity within pathogenic isolates, indicate that the B strains constitute a phylogenetically distinct group within the species.


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