Ferenci Thomas, Spira Beny
School of Molecular and Microbial Biosciences G08, The University of Sydney, NSW 2006, Australia.
Ann N Y Acad Sci. 2007 Oct;1113:105-13. doi: 10.1196/annals.1391.003. Epub 2007 May 4.
Bacteria can exhibit high levels of resistance to one or more environmental stresses such as temperature, osmolarity, radiation, pH, starvation, as well as resistance to noxious chemicals and antibiotics. Yet evolution has not optimized stress resistance in all bacteria to all stresses. Even within a species like Escherichia coli, stress resistance is not constant between strains, suggesting that selection for stress resistance is under counterselection in some environments. The tradeoffs associated with stress resistance in E. coli are due to more than the direct cost of resistance mechanisms. A significant indirect cost is that high stress resistance is associated with a reduced ability to compete for poor growth substrates like acetate or even good substrates like glucose at suboptimal concentrations. High stress resistance also decreases the ability to use inorganic nutrients like phosphate. This tradeoff between self-preservation and nutritional competence, called the SPANC balance, is likely to be the major selective influence in natural populations. Another cost of high stress resistance in E. coli is an elevated mutation rate and the increased generation of deleterious mutations. Directional adaptations in SPANC balance and mutation rate are environment-dependent. The most common variations in SPANC are due to polymorphisms in the levels of global regulators RpoS and ppGpp between different strains. High levels favor stress resistance, and low levels allow better nutrition. The intimate association of RpoS/ppGpp with stress resistance and SPANC balancing influences numerous cellular processes and bacterial properties, including virulence.
细菌可表现出对一种或多种环境压力的高度抗性，如温度、渗透压、辐射、pH值、饥饿，以及对有害化学物质和抗生素的抗性。然而，进化并未使所有细菌对所有压力的抗性都达到最优状态。即使在大肠杆菌这样的物种中，不同菌株之间的抗逆性也并非恒定不变，这表明在某些环境中，对抗逆性的选择处于反向选择之下。大肠杆菌中与抗逆性相关的权衡不仅仅源于抗性机制的直接成本。一个显著的间接成本是，高抗逆性与在诸如乙酸盐等劣质生长底物甚至亚最佳浓度的葡萄糖等优质底物上竞争的能力下降有关。高抗逆性还会降低利用无机营养物如磷酸盐的能力。这种自我保护与营养能力之间的权衡，即所谓的SPANC平衡，可能是自然种群中的主要选择影响因素。大肠杆菌中高抗逆性的另一个代价是突变率升高以及有害突变的产生增加。SPANC平衡和突变率的定向适应取决于环境。SPANC最常见的变化是由于不同菌株之间全局调节因子RpoS和ppGpp水平的多态性。高水平有利于抗逆性，而低水平则允许更好的营养。RpoS/ppGpp与抗逆性和SPANC平衡的密切关联影响着众多细胞过程和细菌特性，包括毒力。
