Operon fusion of the phase variation switch. A virulence factor in Escherichia coli

A derivative strain of a K-12 isolate of Escherichia coli was selected utilizing the operon fusion technique of Casadaban. This derivative strain (VL 361) was characterized by its ability to synthesize the enzyme beta-galactosidase - the gene product of the inserted lacZ gene - as if it were type 1 fimbriae. Enzyme production was found to behave according to the properties of phase variation, thus mimicking the phase variation seen with type 1 fimbriae. The oscillation between the on-and-off synthesis of enzyme was found to be random, and was not influenced by such environmental factors as temperature, glucose or cyclic AMP. Thus the selection in various media of either fimbriate or non-fimbriate states is probably not genetic in nature, but rather the outgrowth of one type of phase over another.