Institute of Microbiology, ETH Zürich, Zurich, Switzerland.
PLoS Pathog. 2011 Jul;7(7):e1002143. doi: 10.1371/journal.ppat.1002143. Epub 2011 Jul 28.
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.
毒力因子通常会增强病原体的适应性，从而促进传播。然而，大多数病原体适应性的研究都是通过对大量种群的表型进行平均来进行的。在这里，我们通过简单的培养实验分析了沙门氏菌亚种 I 血清型 Typhimurium 表达毒力因子的适应性成本。III 型分泌系统 ttss-1 是引发沙门氏菌腹泻的主要毒力因子，只有一小部分沙门氏菌 Typhimurium 种群表达 ttss-1，产生了一种既有表达 ttss-1（TTSS-1(+)表型）的细胞，也有不表达 ttss-1（TTSS-1(-)表型）的细胞混合物。在这里，我们使用荧光蛋白报告基因在单细胞水平上研究了体外 TTSS-1(+)表型。调控因子 hilA 控制 TTSS-1+个体的比例及其 ttss-1 表达水平。引人注目的是，TTSS-1(+)表型的细胞比 TTSS-1(-)表型的细胞生长得更慢。生长迟缓至少部分归因于 TTSS-1 效应子和/或转位器蛋白的表达。尽管存在这种生长缺陷，但在 LB 分批培养的对数后期生长阶段，TTSS-1(+)亚群的比例从<10%增加到约 60%。这归因于 ttss-1 表达的起始率增加，这是由于在该生长阶段积累的环境线索引起的，如实验数据和数学建模所示。最后，hilA 和 hilD 突变体只形成快速生长的 TTSS-1(-)细胞，在混合培养中比野生型沙门氏菌 Typhimurium 更具竞争优势。我们的数据表明，毒力因子表达在非宿主环境中会带来生长缺陷。这就提出了一个重要的问题，即在宿主感染期间，是否有补偿机制来确保基因型的成功传播。
