A better understanding of how antibiotic exposure impacts the evolution of resistance in bacterial populations is crucial for designing more sustainable treatment strategies. The conventional approach to this question is to measure the range of concentrations over which resistant strain(s) are selectively favored over a sensitive strain. Here, we instead investigate how antibiotic concentration impacts the initial establishment of resistance from single cells, mimicking the clonal expansion of a resistant lineage following mutation or horizontal gene transfer. Using two Pseudomonas aeruginosa strains carrying resistance plasmids, we show that single resistant cells have <5% probability of detectable outgrowth at antibiotic concentrations as low as one-eighth of the resistant strain’s minimum inhibitory concentration (MIC). This low probability of establishment is due to detrimental effects of antibiotics on resistant cells, coupled with the inherently stochastic nature of cell division and death on the single-cell level, which leads to loss of many nascent resistant lineages. Our findings suggest that moderate doses of antibiotics, well below the MIC of resistant strains, may effectively restrict de novo emergence of resistance even though they cannot clear already-large resistant populations.

更好地了解抗生素暴露如何影响细菌群体耐药性的进化对于设计更可持续的治疗策略至关重要。解决这个问题的传统方法是测量耐药菌株相对于敏感菌株有选择性的浓度范围。在这里，我们研究抗生素浓度如何影响单细胞耐药性的初步建立，模拟突变或水平基因转移后耐药谱系的克隆扩张。使用两种携带耐药质粒的铜绿假单胞菌菌株，我们发现，在抗生素浓度低至耐药菌株最小抑制浓度 (MIC) 八分之一的情况下，单个耐药细胞有 <5% 的概率可检测到生长。这种低建立概率是由于抗生素对耐药细胞的有害影响，加上单细胞水平上细胞分裂和死亡固有的随机性，导致许多新生耐药谱系的丧失。我们的研究结果表明，中等剂量的抗生素（远低于耐药菌株的 MIC）可以有效限制耐药性的从头出现，即使它们无法清除已经很大的耐药群体。