Comparative transcriptome combined with morphophysiological analyses revealed the molecular mechanism underlying <i>Tetrahymena thermophila</i> predation-induced antiphage defense in <i>Aeromonas hydrophila</i>

Protozoan predation has been demonstrated to be a strong driving force for bacterial defence strategies in the environment. Our previous study demonstrated that Aeromonas hydrophila NJ-35, which evolved small-colony variants (SCVs), displayed various adaptive traits in response to Tetrahymena thermophila predation, such as enhanced phage resistance. However, the evolutionary mechanisms are largely unknown. In this study, we performed a genome- and transcriptome-wide analysis of the SCV1, representing one strain of the SCVs, for identification of the genes of mutation and altered expression underlying this phage resistance phenotype. Our study demonstrated that phage resistance caused by T. thermophila predation was due to the downregulation of a flagellar biosynthesis regulator, flhF, in SCV1. Interestingly, we confirmed that phage resistance in SCV1 was not straightforwardly attributable to the absence of flagella but to FlhF-mediated secretion of extracellular protein that hinders phage adsorption. This finding improves our understanding of the mechanisms by which A. hydrophila lowers the susceptibility to phage infection under predation pressure, and highlights an important contribution of bacterium–protozoan interactions in driving the adaptive evolution of pathogens in complex environments.原生动物捕食已被证实是驱动自然环境中细菌防御策略演化的关键动力。本团队前期研究发现，携带有小菌落变异株（small-colony variants, SCVs）表型的嗜水气单胞菌NJ-35，在应对嗜热四膜虫（Tetrahymena thermophila）捕食时展现出多种适应性性状，例如增强的噬菌体抗性。然而该过程背后的进化机制目前仍未明确。本研究针对代表SCVs菌株之一的SCV1开展全基因组与全转录组分析，以鉴定该噬菌体抗性表型背后的突变基因与差异表达基因。研究证实，嗜热四膜虫捕食诱导SCV1产生的噬菌体抗性，源于鞭毛生物合成调控因子flhF的下调表达。值得注意的是，本研究确认SCV1中的噬菌体抗性并非直接源于鞭毛缺失，而是通过FlhF介导的胞外蛋白分泌阻碍噬菌体吸附实现的。这一发现加深了我们对嗜水气单胞菌在捕食压力下降低噬菌体感染易感性的机制理解，并凸显了细菌-原生动物互作在驱动复杂环境中病原体适应性进化过程中的重要作用。