Analysis of a naturally occurring K99⁺ enterotoxigenic Escherichia coli strain that fails to produce K99

A spontaneously occurring field isolate of enterotoxigenic Escherichia coli that was genotypically K99⁺ but phenotypically K99^- was analyzed for the reason that it did not express K99. The defect, which was cis active, was located within an area 5' to the first gene required for K99 biogenesis and was the result of the deletion of a single base pair.