title: The cost of virulence: retarded growth of Salmonella Typhimurium cells expressing type III secretion system 1.
abstract: Virulence factors generally enhance a pathogen's fitness and thereby foster transmission. However, most studies of pathogen fitness have been performed by averaging the phenotypes over large populations. Here, we have analyzed the fitness costs of virulence factor expression by Salmonella enterica subspecies I serovar Typhimurium in simple culture experiments. The type III secretion system ttss-1, a cardinal virulence factor for eliciting Salmonella diarrhea, is expressed by just a fraction of the S. Typhimurium population, yielding a mixture of cells that either express ttss-1 (TTSS-1(+) phenotype) or not (TTSS-1(-) phenotype). Here, we studied in vitro the TTSS-1(+) phenotype at the single cell level using fluorescent protein reporters. The regulator hilA controlled the fraction of TTSS-1+ individuals and their ttss-1 expression level. Strikingly, cells of the TTSS-1(+) phenotype grew slower than cells of the TTSS-1(-) phenotype. The growth retardation was at least partially attributable to the expression of TTSS-1 effector and/or translocon proteins. In spite of this growth penalty, the TTSS-1(+) subpopulation increased from <10% to approx. 60% during the late logarithmic growth phase of an LB batch culture. This was attributable to an increasing initiation rate of ttss-1 expression, in response to environmental cues accumulating during this growth phase, as shown by experimental data and mathematical modeling. Finally, hilA and hilD mutants, which form only fast-growing TTSS-1(-) cells, outcompeted wild type S. Typhimurium in mixed cultures. Our data demonstrated that virulence factor expression imposes a growth penalty in a non-host environment. This raises important questions about compensating mechanisms during host infection which ensure successful propagation of the genotype.
authors.affiliation: Institute of Microbiology, ETH Zürich, Zurich, Switzerland.
authors.forename: Alexander
authors.initials: A
authors.lastname: Sturm
authors.forename: Matthias
authors.initials: M
authors.lastname: Heinemann
authors.forename: Markus
authors.initials: M
authors.lastname: Arnoldini
authors.forename: Arndt
authors.initials: A
authors.lastname: Benecke
authors.forename: Martin
authors.initials: M
authors.lastname: Ackermann
authors.forename: Matthias
authors.initials: M
authors.lastname: Benz
authors.forename: Jasmine
authors.initials: J
authors.lastname: Dormann
authors.forename: Wolf-Dietrich
authors.initials: WD
authors.lastname: Hardt
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D058947:Bacterial Secretion Systems / Q000502:physiology*; D015964:Gene Expression Regulation, Bacterial; D010641:Phenotype; D012480:Salmonella Infections / Q000235:genetics / Q000378:metabolism; D012486:Salmonella typhimurium / Q000235:genetics / Q000254:growth & development / Q000472:pathogenicity; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; C102236:HilA protein, Salmonella; D015534:Trans-Activators; D037521:Virulence Factors