Aw. Paton et al., COMPARATIVE TOXICITY AND VIRULENCE OF ESCHERICHIA-COLI CLONES EXPRESSING VARIANT AND CHIMERIC SHIGA-LIKE TOXIN TYPE-II OPERONS, Infection and immunity, 63(7), 1995, pp. 2450-2458
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, design
ated SLT-II/O111, SLT-II/OX3a, SLT-II/OX3b, and SLT-II/O48, were clone
d in the same orientation in pBluescript. French pressure cell lysates
of E. coli DH5 alpha derivatives carrying these plasmids differed mar
kedly in cytotoxicity for Vero cells, with 50% cytotoxic doses ranging
from 20 to 328,000/ml. The strains also differed in oral virulence fo
r streptomycin-treated mice, as judged by survival rate and/or median
survival time, but virulence did not necessarily correlate with in vit
ro cytotoxicity. The SLT-II type associated with the lowest oral virul
ence was SLT-II/O111. Both the overall survival rate and the median su
rvival time of mice challenged with clones producing this toxin were s
ignificantly greater than that for mice challenged with a clone produc
ing 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-176-->Gly substitution in the mature A sub
unit. Clones producing SLT-II/O48 and SLT-II/OX3b had similarly high c
ytotoxicities for Vero cells, but the latter was more virulent when fe
d to streptomycin-treated mice, as judged by median survival time. Exp
eriments with clones carrying chimeric O48/OX3b SLT-II operons indicat
ed 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 a
mino acids (Met-4-->Thr and Gly-102-->Asp, respectively). These findin
gs raise the possibility that naturally occurring SLT-II sequence vari
ations may impact directly on the capacity of a given SLT-producing E.
coli strain to cause disease.