It is widely assumed that all mutant microorganisms present in a culture are able to grow and form colonies, provided that they express the features required for selection. Unlike wild-type Escherichia coli, PHO-constitutive mutants overexpress alkaline phosphatase and hence can hydrolyze glycerol-2-phosphate (G2P) to glycerol and form colonies on plates having G2P as the sole carbon source. These mutations mostly occur in the pst operon. However, the frequency of PHO-constitutive colonies on the G2P selective plate is exceptionally low. We show that the rate in which spontaneous PHO-constitutive mutations emerge is about 8.0 × 10−6/generation, a relatively high rate, but the growth of most existing mutants is inhibited by their neighboring wild-type cells. This inhibition is elicited only by non-mutant viable bacteria that can take up and metabolize glycerol formed by the mutants. Evidence indicates that the few mutants that do form colonies derive from microclusters of mutants on the selective plate. A mathematical model that describes the fate of the wild-type and mutant populations under these circumstances supports these results. This scenario in which neither the wild-type nor the majority of the mutants are able to grow resembles an unavoidable “tragedy of the commons” case which results in the collapse of the majority of the population. Cooperation between rare adjacent mutants enables them to overcome the competition and eventually form mutant colonies. The inhibition of PHO-constitutive mutants provides an example of mutant frequency masked by orders of magnitude due to a competition between mutants and their ancestral wild-type cells. Similar “tragedy of the commons-like” cases may occur in other settings and should be taken into consideration while estimating true mutant frequencies and mutation rates.

中文翻译：

---

营养资源竞争掩盖了细菌突变体的真实频率

普遍假设存在于培养物中的所有突变微生物都能够生长并形成菌落，只要它们表达选择所需的特征即可。与野生型大肠杆菌不同，PHO组成型突变体过表达碱性磷酸酶，因此可以将2-磷酸甘油酯（G2P）水解为甘油，并在以G2P作为唯一碳源的板上形成菌落。这些突变主要发生在pst操纵子中。但是，G2P选择性板上PHO组成型菌落的频率异常低。我们表明，自发的PHO组成型突变出现的速率约为8.0×10-6 /代，是一个相对较高的速率，但是大多数现有突变体的生长受到其邻近的野生型细胞的抑制。这种抑制作用仅由能吸收并代谢突变体形成的甘油的非突变活细菌引起。证据表明，确实形成菌落的少数突变体来自选择性板上突变体的微团簇。描述这种情况下野生型和突变型种群命运的数学模型支持了这些结果。野生型或大多数突变体都无法生长的这种情况类似于不可避免的“公地悲剧”，导致大多数人口崩溃。稀有相邻突变体之间的合作使他们能够克服竞争并最终形成突变体菌落。由于突变体与其祖先野生型细胞之间的竞争，对PHO组成型突变体的抑制提供了一个被数量级掩盖的突变体频率实例。在其他情况下也可能发生类似的“类似公地悲剧”的情况，在估算真实的突变频率和突变率时应予以考虑。