Phase variation of F165(1) (Prs-like) fimbriae from Escherichia coli causing septicaemia in animals

Escherichia coli O115:F165 strains are associated with septicaemia in young pigs and synthesize fimbriae involved in virulence, designated as F165(1). F165(1) fimbriae belong to the P fimbrial family and are encoded by the fo o gene cluster. The foo regulatory region of strain 5131 possesses characte ristics similar to that of members of the P regulatory family, including pa pI and papB homologues, and two GATC sites separated by 102 bp, targets of differential Dam methylation. In wild-type strains, the synthesis of F165(1 ) is repressed by leucine and the fimbriae undergo phase variation. Immunof luorescence staining showed that phase variation of F165(1) results in a ma jority of cells (98%) in the ON phase, in contrast with phase variation of other members of this regulatory family, for which the majority of the cell s are in the OFF state. Using a translational fusion in strain 5131 between phoA and fooA, encoding for the major structural subunit of F165(1), it wa s shown that leucine inhibits the OFF to ON switch and modulates the basal transcription of the foo operon.