Micalis Institute, INRA, AgroParisTech, Université Paris-Saclay, 78350 Jouy-en-Josas, France.
INRA, UMR 203, 11 Av J. Capelle, 69621 Villeurbanne, France.
Sci Rep. 2016 Jul 20;6:29349. doi: 10.1038/srep29349.
Production of reactive nitrogen species (NO) is a key step in the immune response following infections. NO induces lesions to bacterial DNA, thus limiting bacterial growth within hosts. Using two pathogenic bacteria, Bacillus cereus and Shigella flexneri, we show that the DNA-repair protein Mfd (Mutation-Frequency-Decline) is required for bacterial resistance to the host-NO-response. In both species, a mutant deficient for mfd does not survive to NO, produced in vitro or by phagocytic cells. In vivo, the ∆mfd mutant is avirulent and unable to survive the NO-stress. Moreover, NO induces DNA-double-strand-breaks and point mutations in the Δmfd mutant. In overall, these observations demonstrate that NO damages bacterial DNA and that Mfd is required to maintain bacterial genomic integrity. This unexpected discovery reveals that Mfd, a typical housekeeping gene, turns out to be a true virulence factor allowing survival and growth of the pathogen in its host, due to its capacity to protect the bacterium against NO, a key molecule of the innate immune defense. As Mfd is widely conserved in the bacterial kingdom, these data highlight a mechanism that may be used by a large spectrum of bacteria to overcome the host immune response and especially the mutagenic properties of NO.
活性氮物种（NO）的产生是感染后免疫反应的关键步骤。NO 会导致细菌 DNA 损伤，从而限制宿主内细菌的生长。我们使用两种致病菌，蜡样芽孢杆菌和福氏志贺菌，表明 DNA 修复蛋白 Mfd（突变频率下降）对于细菌抵抗宿主的 NO 反应是必需的。在这两种细菌中，mfd 缺陷型突变体无法在体外或吞噬细胞产生的 NO 中存活。在体内，Δmfd 突变体毒力丧失，无法在 NO 压力下存活。此外，NO 会导致 Δmfd 突变体的 DNA 双链断裂和点突变。总之，这些观察结果表明，NO 会损伤细菌 DNA，而 Mfd 是维持细菌基因组完整性所必需的。这一意外发现表明，Mfd 是一种典型的管家基因，由于其能够保护细菌免受 NO 的侵害，而 NO 是先天免疫防御的关键分子，因此它实际上是一种毒力因子，使病原体能够在其宿主中存活和生长。由于 Mfd 在细菌王国中广泛保守，这些数据突出了一种机制，该机制可能被广泛的细菌用于克服宿主免疫反应，特别是 NO 的诱变特性。
