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Journal of Bacteriology, December 2006, p. 8395-8406, Vol. 188, No. 24
0021-9193/06/$08.00+0     doi:10.1128/JB.00798-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Comparative Genetics of the rdar Morphotype in Salmonella{triangledown}

A. P. White1 and M. G. Surette1,2*

Department of Microbiology and Infectious Diseases,1 Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, Alberta T2N 4N1, Canada2

Received 4 June 2006/ Accepted 24 September 2006

The Salmonella rdar morphotype is a distinct, rough and dry colony morphology formed by the extracellular interaction of thin aggregative fimbriae (Tafi or curli), cellulose, and other polysaccharides. Cells in rdar colonies are more resistant to desiccation and exogenous stresses, which is hypothesized to aid in the passage of pathogenic Salmonella spp. between hosts. Here we analyzed the genetic and phenotypic conservation of the rdar morphotype throughout the entire Salmonella genus. The rdar morphotype was conserved in 90% of 80 isolates representing all 7 Salmonella groups; however, the frequency was only 31% in a reference set of 16 strains (Salmonella reference collection C [SARC]). Comparative gene expression analysis was used to separate cis- and trans-acting effects on promoter activity for the 16 SARC strains, focusing on the 780-bp intergenic region containing divergent promoters for the master regulator of the rdar morphotype (agfD) and the Tafi structural genes (agfB). Surprisingly, promoter functionality was conserved in most isolates, and loss of the phenotype was due primarily to defects in trans-acting regulatory factors. We hypothesize that trans differences have been caused by domestication, whereas cis differences, detected for Salmonella enterica subsp. arizonae isolates, may reflect an evolutionary change in lifestyle. Our results demonstrate that the rdar morphotype is conserved throughout the salmonellae, but they also emphasize that regulation is an important source of variability among isolates.

* Corresponding author. Mailing address: Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, Alberta T2N 4N1, Canada. Phone: (403) 220-2744. Fax: (403) 270-2772. E-mail: surette{at}ucalgary.ca.

{triangledown} Published ahead of print on 29 September 2006.

Journal of Bacteriology, December 2006, p. 8395-8406, Vol. 188, No. 24
0021-9193/06/$08.00+0     doi:10.1128/JB.00798-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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