CONDITIONAL LYSIS OF ESCHERICHIA-COLI BY THE FUSION OF EXTRACELLULAR (STA) TO PERIPLASMIC (LTB) ENTEROTOXINS - APPARENT PHENOTYPIC SUPPRESSION OF LACTOSE PERMEASE

The expression of a methanol-soluble, heat-stable enterotoxin (STA) fused to the B subunit of the heat-labile enterotoxin (LTB) at 35°C or higher temperatures caused strains of Escherichia coli deficient in lactose permease to behave on indicator media as Lac+; however, at 33°C or lower temperatures the original Lac- phenotype of the host strains was maintained. The apparent phenotypic suppression of lacY was shown to be due to lysis of a fraction of the bacteria and the consequent release of active β-galactosidase to the culture supernatant. After incubation at 37°C for 1 h, the cultures were committed to lyse. Plasmid and chromosomal mutants that do not show this phenotype were isolated by selecting Lac- colonies at the unpermissive temperature. The mutations on the plasmids were localized in both the heat-stable and the heat-labile enterotoxin genes. Chromosomal mutants that show normal levels of β-galactosidase and fused toxins have also been isolated. © 1990 Springer-Verlag New York Inc.