Toxin detection after storage or cultivation of enterotoxigenic with colicinogenic Escherichia coli: a possible mechanism for toxin-negative pools.

Of 100 non-enterotoxigenic Escherichia coli isolated from children with diarrhea in Bangkok, Thailand, 24 were found to produce colicin(s). Of these, 87% were active against one or more enterotoxigenic E. coli isolated from the same population. Storage of nine enterotoxigenic E. coli with known inhibitory colicin-producing E. coli in different proportions caused 51 of 96 pools to become negative in the suckling mouse assay (heat-stable toxin) and 17 of 52 to become negative in the Y-1 adrenal cell assay (heat-labile toxin). Cocultivation of the same strains, without prior storage, caused 12 of 96 pools to become negative for heat-stable toxin and 1 of 52 pools to become negative for heat-labile toxin. Storage or cultivation of E. coli in pools may cause negative results in the suckling mouse and Y-1 adrenal cell assays if any of the isolates in the pool produces colicin(s).