Thin Aggregative Fimbriae and Cellulose Enhance Long-Term Survival and Persistence ofSalmonella

Abstract

Salmonella spp. are environmentally persistent pathogens that have served as one of the important models for understanding how bacteria adapt to stressful conditions. However, it remains poorly understood how they survive extreme conditions encountered outside their hosts. Here we show that the rdar morphotype, a multicellular phenotype characterized by fimbria-and cellulose-mediated colony pattern formation, enhances the resistance of Salmonella to desiccation. When colonies were stored on plastic for several months in the absence of exogenous nutrients, survival of wild-type cells was increased compared to mutants deficient in fimbriae and/or cellulose production. Differences between strains were further highlighted upon exposure to sodium hypochlorite, as cellulose-deficient strains were 1,000-fold more susceptible. Measurements of gene expression using luciferase reporters indicated that production of thin aggregative fimbriae (Tafi) may initiate formation of colony surface patterns characteristic of the rdar morphotype. We hypothesize that Tafi play a role in the organization of different components of the extracellular matrix. Conservation of the rdar morphotype among pathogenic S. enterica isolates and the survival advantages that it provides collectively suggest that this phenotype could play a role in the transmission of Salmonella between hosts.

Comparative Genetics of the rdar Morphotype inSalmonella

“…Conservation of Tafi production and rdar morphotype formation in these other subgroups suggests an important role in the life cycles of diverse Salmonella isolates. Due to characteristic growth conditions (growth under 30°C, nutrient limitation, low osmolarity) (16,25) and increased resistance to desiccation (26,68) and antimicrobial agents (2,55,56,68), we hypothesize that the rdar morphotype has a fundamental role for Salmonella survival outside host environments. This may contribute to contamination of different food products (4,30) and passage between susceptible hosts.…”

“…Template DNA from each Salmonella strain was prepared following the method of Walsh et al (64). agfB and agfD, mlrA, or rpoS promoter-containing fragments were PCR amplified using primers agfD1 and agfD2 (68) (36). Since BapA (38) and capsular polysaccharides (26) were only recently discovered, their precise roles in multicellular aggregation are not well understood.…”

“…The RpoS-responsive synthetic promoter::lux fusion sig38-H4 has been previously described (68). Plasmids were transformed into all Salmonella strains by electroporation (Gene Pulser 2.0; Bio-Rad Laboratories Inc.).…”

“…Primary control takes place within the large intergenic region between the master regulator, agfD, and the Tafi structural genes, agfBAC. Transcription of agfD is dependent upon the stationary-phaseinducible sigma factor RpoS, and is maximal in the late exponential or early stationary phase of growth (25,52,68). Crl acts as a cofactor in this process by stimulating the binding of the RpoS-RNA polymerase complex to agf promoter regions (9,48).…”

“…Extracellular matrix production is associated with multicellular properties of Salmonella, both in the formation of the rdar morphotype (16,52) and in the formation of pellicles at the air-liquid interface in standing culture (51,56). These phenotypes enhance the resistance of Salmonella to antimicrobial stresses (2,55) and contribute to long-term survival (68).…”