Comparative toxicity and virulence of Escherichia coli clones expressing variant and chimeric Shiga-like toxin type II operons

Abstract

Shiga-like toxin (SLT)-producing strains of Escherichia coli are known to cause diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome in humans. The SLTs, particularly those related to type II (SLT-II), are a diverse family of toxins which may have differing in vitro or in vivo properties. To examine the impact of naturally occurring SLT-II sequence variation on the capacity of a given E. coli strain to cause disease, operons encoding four different SLT-II-related toxins, designated SLT-II/O111, SLT-II/OX3a, SLT-II/OX3b, and SLTII/ O48, were cloned in the same orientation in pBluescript. French pressure cell lysates of E. coli DH5a derivatives carrying these plasmids differed markedly in cytotoxicity for Vero cells, with 50% cytotoxic doses ranging from 20 to 328,000/ml. The strains also differed in oral virulence for streptomycin-treated mice, as judged by survival rate and/or median survival time, but virulence did not necessarily correlate with in vitro cytotoxicity. The SLT-II type associated with the lowest oral virulence was SLT-II/O111. Both the overall survival rate and the median survival time of mice challenged with clones producing this toxin were significantly greater than that for mice challenged with a clone producing the closely related SLT-II/OX3a. Experiments with clones carrying chimeric O111/OX3a SLT-II operons indicated that the reduced virulence was associated with an Arg-1763Gly substitution in the mature A subunit. Clones producing SLT-II/O48 and SLT-II/OX3b had similarly high cytotoxicities for Vero cells, but the latter was more virulent when fed to streptomycin-treated mice, as judged by median survival time. Experiments with clones carrying chimeric O48/OX3b SLT-II operons indicated that the increased virulence was a function of the A subunit of SLT-II/ OX3b, which differs from the A subunit of SLT-II/O48 by only two amino acids (Met-43Thr and Gly-1023Asp, respectively). These findings raise the possibility that naturally occurring SLT-II sequence variations may impact directly on the capacity of a given SLT-producing E. coli strain to cause disease.