The seventh
cholera pandemic that started in 1961 was caused by Vibrio cholerae O1 strains of the El Tor biotype. These strains produce the pore-forming toxin
hemolysin, a characteristic used clinically to distinguish classical and El Tor biotypes. Even though extensive in vitro data on the cytolytic activities of
hemolysin exist, the connection of
hemolysin to virulence in vivo is not well characterized. To study the contribution of
hemolysin and other accessory toxins to pathogenesis, we utilized the model of intestinal
infection in adult mice sensitive to the actions of accessory toxins. In this study, we showed that 4- to 6-week-old
streptomycin-fed C57BL/6 mice were susceptible to intestinal
infection with El Tor strains, which caused rapid death at high doses.
Hemolysin had the predominant role in lethality, with a secondary contribution by the multifunctional autoprocessing RTX (MARTX) toxin.
Cholera toxin and
hemagglutinin/protease did not contribute to lethality in this model. Rapid death was not caused by increased dissemination due to a damaged epithelium since the numbers of CFU recovered from spleens and livers 6 h after
infection did not differ between mice inoculated with
hemolysin-expressing strains and those infected with non-
hemolysin-expressing strains. Although accessory toxins were linked to virulence, a strain defective in the production of accessory toxins was still immunogenic since mice immunized with a multitoxin-deficient strain were protected from a subsequent lethal challenge with the wild type. These data suggest that
hemolysin and MARTX toxin contribute to
vaccine reactogenicity but that the genes for these toxins can be deleted from
vaccine strains without affecting
vaccine efficacy.