GENETIC STRUCTURES OF THE B-2 AND B-1 ESCHERICHIA-COLI STRAINS RESPONSIBLE FOR EXTRAINTESTINAL INFECTIONS

Escherichia coli strains causing human extra-intestinal infections may be divided into two groups, B-1 and B-2 according to the electrophore tic patterns of carboxylesterase B. This study compares the restrictio n fragment length polymorphism (RFLP) of ribosomal DNA (rDNA) for 45 B -1 strains and 45 B-2 strains to examine the genetic structure of B-2 strains and to distinguish them from B-1 strains. The isolates were ch osen for diversity in their allozymes of esterases, B, A, C and I, the ir production of virulence factors (alpha-haemolysin, mannose resistan t haemagglutinin and cytotoxic necrotizing factor) and certain O antig ens, and their pathological and geographical origins. DNA was digested with HindIII and BamHI restriction enzymes and analysed by Southern b lotting. The resulting rDNA RFLP patterns of B-2 strains were distinct from those of the B-1 strains. Moreover, the B-2 strains appeared to be less heterogeneous than the B-1 strains. The B-2 strains gave 13 ri botypes (resulting from the combination of the rDNA RFLP patterns obta ined with HirtdIII and BamHI digestions) while the B-1 strains gave 32 ribotypes. Correspondence analysis of the data showed that several cl usters of strains were identified in the B-2 strains by particular rib otypes, certain associations of esterase B and A electrophoretic varia nts, O serotypes and virulence factor production. In contrast, these p arameters appeared to be unrelated in the B-1 strains, reflecting thei r heterogeneity. These findings, which differentiate two levels of gen etic heterogeneity within E. coli pathogenic isolates, indicate that t he B-2 strains constitute a phylogenetically distinct group within the species.