Molecular Comparison of Plasmids Encoding Heat-Labile Enterotoxin Isolated from Escherichia coli Strains of Human Origin

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Abstract
The molecular properties of enterotoxin (Ent) plasmids from 12 E. coli strains of human origin were examined. Ten strains belonged to the O78 serogroup, and the remainder were of serogroup O7 or O159. Eleven plasmids coded for heat-labile enterotoxin (LT), and 1 coded for heat-stable enterotoxin (ST) and LT. The results of restriction enzyme digests and DNA reassociation experiments showed that all the Ent plasmids were related, and supported the subdivision of the LT plasmids into 3 groups based on their genetic properties. Within group 1, 2 plasmids from South African strains were indistinguishable but differed in EcoRI and HindIII digests from the LT plasmid that originated from an Ethiopian strain. The 3 plasmids had > 70% homology. The 2 non-autotransferring group 2 plasmids identified in O78.H11 strains from Bangladesh were indistinguishable. The group 3 plasmids were from strains belonging to serogroups O7 and O78 isolated in Bangladesh, India and Thailand. They shared > 95% homology but showed slight differences in fragment patterns when treated with EcoRI and HindIII. There was 60-70% homology between the plasmids of groups 1 and 3, and the group 2 plasmid had 40-50% homology with members of these 2 groups. The autotransferring Ent plasmids had up to 40% homology with R factors of incompatibility groups FI, FII and FIV.

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