Thermoactivation of a periplasmic heat-stable enterotoxin of Escherichia coli

Strains of Escherichia coli that host a plasmid that codes for the heat-stable (ST) enterotoxin showed 160 times more extracellular enterotoxin than intracellular activity. However, when washed bacteria were sonicated and incubated at between 50 and 85 degrees C, an activity similar to that of the ST enterotoxin was detected. No such effect was present in strains lacking the plasmid, in a plasmid ST- mutant, or in chromosomal mutants that lack a cyclic AMP-linked positive regulatory system which previously were shown to yield an ST- phenotype. The thermoactivation was inhibited by iodoacetamide and N-ethylmaleimide; chloramphenicol did not affect the phenomenon. The heat-activated ST-like enterotoxin was localized in the periplasmic space. The results are discussed in relation to the export of the toxin from the periplasm to the outside of the cell.