Evolutionary loss of the rdar morphotype in Salmonella as a result of high mutation rates during laboratory passage

Abstract

Rapid evolution of microbes under laboratory conditions can lead to domestication of environmental or clinical strains. In this work, we show that domestication due to laboratory passage in rich medium is extremely rapid. Passaging of wild-type Salmonella in rich medium led to diversification of genotypes contributing to the loss of a spatial phenotype, called the rdar morphotype, within days. Gene expression analysis of the rdar regulatory network demonstrated that mutations were primarily within rpoS, indicating that the selection pressure for scavenging during stationary phase had the secondary effect of impairing this highly conserved phenotype. If stationary phase was omitted from the experiment, radiation of genotypes and loss of the rdar morphotype was also demonstrated, but due to mutations within the cellulose biosynthesis pathway and also in an unknown upstream regulator. Thus regardless of the selection pressure, rapid regulatory changes can be observed on laboratory timescales. The speed of accumulation of rpoS mutations during daily passaging could not be explained by measured fitness and mutation rates. A model of mutation accumulation suggests that to generate the observed accumulation of r 38 mutations, this locus must experience a mutation rate of approximately 10 À4 mutations/gene/generation. Sequencing and gene expression of population isolates indicated that there were a wide variety of r 38 phenotypes within each population. This suggests that the rpoS locus is highly mutable by an unknown pathway, and that these mutations accumulate rapidly under common laboratory conditions.

Salmonella biofilms: An overview on occurrence, structure, regulation and eradication

“…Next to the two clear sequence (cis) mutations, generating inactive csgB and csgD promoters, reflecting a different evolutionary lifestyle (no or minimal non-host environmental passage during lifecycle), six other isolates harboured upstream regulatory (trans) mutations, responsible for rdar phenotype loss, probably originating from domestication. In line with this, recent evidence indeed showed that rapid domestication due to laboratory passage in rich medium was responsible for the evolutionary loss of the rdar morphotype (Davidson, White, & Surette, 2008). This loss most of the times appeared to be related to mutations in rpoS, within the cellulose biosynthesis pathway or in unknown upstream rdar morphotype regulators.…”

Bistable Expression of CsgD in Salmonella enterica Serovar Typhimurium Connects Virulence to Persistence

“…For pathogens like S. Typhimurium, the maintenance of a subset of cells primed for invasion but not ideally suited for environmental survival would impose a penalty unless a host is encountered. The energy commitment required to make SPI-1 T3SS organelles (82) or extracellular matrix polymers (83,84) in different subpopulations indicates that it might be a strategy for S. Typhimurium to prepare for unpredictability. A proportion of cells could survive exposure to either the host or nonhost environment, with the ultimate goal being preservation of the shared genome for the next generation (54,80).…”

Bistable Expression of CsgD in Biofilm Development ofSalmonella entericaSerovar Typhimurium

“…3D and 4B). The energy theory arises from the observation that the capability to express multicellular behavior is easily lost under laboratory conditions (8,34). Although there is no doubt that production of extracellular matrix is energetically expensive, another reason for phenotypic heterogeneity can be to maintain the developmental potential of the population.…”