Shigella enterotoxin 1: an enterotoxin of Shigella flexneri 2a active

Culture filtrates of Shigellaflexneri 2a strain M4243 grown in iron-depleted medium, caused significant fluid accumula-tion in rabbit ileal loops. Also, when tested in Ussing cham-bers, a greater rise in potential difference and short circuit current was seen with such filtrates compared with the me-dium control. Analogous filtrates from two M4243 deriva-tives lacking the 140-MD invasiveness plasmid (either M4243avir or BS103) retained 60-65 % of the wild-type enterotoxic activity. Ultrafiltration and gel exclusion size fractionation ofM4243 filtrate revealed that the activity was 60 kD. SDS-PAGE performed on this fraction showed 18 bands, 5 of which reacted with human convalescent sera. Genes encoding this enterotoxin, named ShET1 for Shigella enterotoxin 1, were cloned from the S. flexneri 2a chromo-some, and two separate open reading frames of 534 and 186 bp were sequenced. These observations suggest that S. flexneri 2a elaborates two distinct enterotoxins: ShET1, en-coded by genes located on the chromosome, and ShET2, encoded by a gene on the 140-MD invasiveness plasmid. ShET1, which is composed of two distinct subunits and is elaborated in vivo, where it elicits an immune response, may be important in the pathogenesis of diarrheal illness due t