Farrell Michael J, Finkel Steven E
Molecular and Computation Biology Program, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-1340, USA.
J Bacteriol. 2003 Dec;185(24):7044-52. doi: 10.1128/JB.185.24.7044-7052.2003.
Escherichia coli cells that are aged in batch culture display an increased fitness referred to as the growth advantage in stationary phase, or GASP, phenotype. A common early adaptation to this culture environment is a mutant rpoS allele, such as rpoS819, that results in attenuated RpoS activity. However, it is important to note that during long-term batch culture, environmental conditions are in flux. To date, most studies of the GASP phenotype have focused on identifying alleles that render an advantage in a specific environment, Luria-Bertani broth (LB) batch culture. To determine what role environmental conditions play in rendering relative fitness advantages to E. coli cells carrying either the wild-type or rpoS819 alleles, we performed competitions under a variety of culture conditions in which either the available nutrients, the pH, or both were manipulated. In LB medium, we found that while the rpoS819 allele confers a strong competitive fitness advantage at basic pH, it confers a reduced advantage under neutral conditions, and it is disadvantageous under acidic conditions. Similar results were found using other media. rpoS819 conferred its greatest advantage in basic minimal medium in which either glucose or Casamino Acids were the sole source of carbon and energy. In acidic medium supplemented with either Casamino Acids or glucose, the wild-type allele conferred a slight advantage. In addition, populations were dynamic under all pH conditions tested, with neither the wild-type nor mutant rpoS alleles sweeping a culture. We also found that the strength of the fitness advantage gained during a 10-day incubation is pH dependent.
在分批培养中老化的大肠杆菌细胞表现出适应性增强，即所谓的稳定期生长优势（GASP）表型。对这种培养环境的一种常见早期适应是突变的rpoS等位基因，如rpoS819，其导致RpoS活性减弱。然而，需要注意的是，在长期分批培养过程中，环境条件是不断变化的。迄今为止，大多数关于GASP表型的研究都集中在鉴定在特定环境（即LB肉汤分批培养）中具有优势的等位基因。为了确定环境条件在赋予携带野生型或rpoS819等位基因的大肠杆菌细胞相对适应性优势方面所起的作用，我们在各种培养条件下进行了竞争实验，其中对可用营养物质、pH值或两者进行了调控。在LB培养基中，我们发现虽然rpoS819等位基因在碱性pH条件下赋予强大的竞争适应性优势，但在中性条件下优势降低，在酸性条件下则不利。使用其他培养基也发现了类似结果。rpoS819在以葡萄糖或酪蛋白氨基酸作为唯一碳源和能源的碱性基本培养基中优势最大。在添加了酪蛋白氨基酸或葡萄糖的酸性培养基中，野生型等位基因具有轻微优势。此外，在所有测试的pH条件下，群体都是动态的，野生型和突变型rpoS等位基因都没有在培养物中占据主导地位。我们还发现，在10天培养期间获得的适应性优势强度取决于pH值。
