MULTICELLULAR AND AGGREGATIVE BEHAVIOR OF SALMONELLA-TYPHIMURIUM STRAINS IS CONTROLLED BY MUTATIONS IN THE AGFD PROMOTER

A colony morphology type is described in which cells of Salmonella typ himurium form a rigid multicellular network with expression of thin ag gregative fimbriae that mediate tight intercellular bonds. Surface tra nslocation of cells on plates and adherence to glass and polystyrene s urfaces in biofilm assays are further characteristics of the morphotyp e. This morphotype (rdar) is normally expressed only at low temperatur e. However, in two unrelated S. typhimurium strains, spontaneous mutan ts were found forming rdar colonies independent of temperature. Alleli c replacement proved a single point mutation in the promoter region of PagfD in each of the two mutants to be responsible for the constituti ve phenotype of a multicellular behaviour. Transcription levels of the two divergently transcribed agf operons required for biogenesis of th in aggregative fimbriae by Northern blot analysis with gene probes for agfA and agfD as well as expression levels of AgfA by Western blottin g were compared in the wild type, the constitutive mutants and their r espective ompR(-) and rpoS(-) derivatives. In the wild type the rdar m orphotype and expression of thin aggregative fimbriae are restricted t o low temperature on plates containing rich medium of low osmolarity, but biogenesis of thin aggregative fimbriae occurs upon iron starvatio n at 37 degrees C. In the upregulated mutants biogenesis of thin aggre gative fimbriae is only abolished at high osmolarity at 37 degrees C a nd in the exponential phase in broth culture. Control of expression of thin aggregative fimbriae and rdar morphology takes place at the tran scriptional level at the agfD promoter. A functional ompR allele is re quired, however an rpoS mutation abolishes transcription only in the w ild type, but has no influence on expression of thin aggregative fimbr iae in the constitutive mutants.