The red arrows depict possible evolutionary paths of a novel colonizer along adaptive peaks in the intestinal fitness landscapes that change with the status of the host immune system. The flat surfaces represent the non-null fitness baselines (values x or y) at which a bacterium can establish at minimum carrying capacity. a In the healthy gut, metabolic competence, resistance to aggressions by competitors and predators, swift adaptation to rapid fluctuations as well as surviving acidic pH and the flow of the intestinal content, represent potent selective pressures and as many opportunities for bacteria to increase fitness by phenotypic or genetic variations. b When pathogens trigger acute inflammation, bacteria must adapt to iron starvation, killing by immune cells and antimicrobial peptides, and oxidative stress, while new metabolic opportunities emerge. c When high-affinity SIgA are produced against a bacterium, e.g., after oral vaccination, escape of SIgA by altering or losing surface epitopes becomes crucial for maximum fitness. However, escaping polyvalent SIgA responses after vaccination with “evolutionary trap” vaccines leads to evolutionary trade-offs: A fitness maximum is reached in the vaccinated host gut that represents a major disadvantage for transmission into naïve hosts (fitness diminished below x) (d).

红色箭头描绘了新型殖民者沿着肠道适应度峰值的可能进化路径，该峰值随宿主免疫系统的状态而变化。平坦表面代表非零适应度基线（值 x 或 y），在该基线上细菌可以建立最小承载能力。 a在健康的肠道中，代谢能力、对竞争者和捕食者攻击的抵抗力、对快速波动的快速适应以及酸性 pH 值的生存和肠道内容物的流动，代表着强大的选择压力，并且为细菌提供了许多通过以下方式提高适应性的机会：表型或遗传变异。 b当病原体引发急性炎症时，细菌必须适应铁饥饿、免疫细胞和抗菌肽的杀伤以及氧化应激，同时出现新的代谢机会。 c当针对细菌产生高亲和力 SIgA 时，例如口服疫苗接种后，通过改变或丢失表面表位来逃逸 SIgA 对于最大适应性变得至关重要。然而，在接种“进化陷阱”疫苗后逃避多价 SIgA 反应会导致进化权衡：接种疫苗的宿主肠道达到了适应度最大值，这对于传播到幼稚宿主来说是一个主要缺点（适应度降低到 x 以下）（ d ） 。