INCREASED ORAL VIRULENCE OF ESCHERICHIA-COLI EXPRESSING A VARIANT SHIGA-LIKE TOXIN TYPE-II OPERON IS ASSOCIATED WITH BOTH A-SUBUNIT RESIDUES MET4 AND GLY102

We have previously demonstrated that Escherichia coli DH5 alpha clones expressing closely-related Shiga-like toxin type II operons (designat ed SLT-II/OX3b and SLT-II/048) had similar cytotoxicity for Vero cells , but differed in oral virulence for streptomycin-treated mice. Studie s with chimeric toxin operons indicated that increased virulence was a ssociated with the A subunit of SLT-II/OX3b, which differs from that o f SLT-II/048 by two amino acids (at positions 4 and 102). In the prese nt study, we have constructed a series of additional chimeric derivati ves of the SLT-II/OX3b and SLT-II/048 operons and assessed the effect of single A subunit amino acid substitutions on oral virulence. Maxima l virulence, as judged by median survival time after oral challenge, w as associated only with the combination of Met4 and Gly102, as found i n the A subunit of SLT-II/OX3b. (C) 1995 Academic Press Limited