Vibrio fischeri lux Genes Play an Important Role in Colonization and Development of the Host Light Organ

The bioluminescent bacterium Vibrio fischeri and juveniles of the squid Euprymna scolopes specifically recognize and respond to one another during the formation of a persistent colonization within the host's nascent light-emitting organ. The resulting fully developed light organ contains brightly luminescing bacteria and has undergone a bacterium-induced program of tissue differentiation, one component of which is a swelling of the epithelial cells that line the symbiont-containing crypts. While the luminescence (lux) genes of symbiotic V. fischeri have been shown to be highly induced within the crypts, the role of these genes in the initiation and persistence of the symbiosis has not been rigorously examined. We have constructed and examined three mutants (luxA, luxI, and luxR), defective in either luciferase enzymatic or regulatory proteins. All three are unable to induce normal luminescence levels in the host and, 2 days after initiating the association, had a three- to fourfold defect in the extent of colonization. Surprisingly, these lux mutants also were unable to induce swelling in the crypt epithelial cells. Complementing, in trans, the defect in light emission restored both normal colonization capability and induction of swelling. We hypothesize that a diminished level of oxygen consumption by a luciferase-deficient symbiotic population is responsible for the reduced fitness of lux mutants in the light organ crypts. This study is the first to show that the capacity for bioluminescence is critical for normal cell-cell interactions between a bacterium and its animal host and presents the first examples of V. fischeri genes that affect normal host tissue development.

费氏弧菌（Vibrio fischeri）与夏威夷短尾乌贼（Euprymna scolopes）幼体在宿主新生发光器官内建立持续性定殖的过程中，二者会发生特异性识别并相互应答。发育完全的发光器官中富集明亮发光的细菌，且经历了由细菌诱导的组织分化程序，其中一项标志性变化为含共生体的隐窝内衬上皮细胞发生肿胀。现有研究已证实，共生费氏弧菌的发光（lux）基因在隐窝内被高度诱导，但此类基因在共生关系的起始与维持过程中的具体作用尚未得到严谨验证。本研究构建并分析了三种突变体（luxA、luxI及luxR），它们分别存在荧光素酶酶活缺陷或调控蛋白功能缺陷。三类突变体均无法在宿主中诱导正常发光水平，且在定殖起始2天后，其定殖程度较野生型降低3至4倍。令人意外的是，这些lux突变体同样无法诱导隐窝上皮细胞发生肿胀。通过反式互补修复发光缺陷后，突变体的正常定殖能力与细胞肿胀诱导能力均得以恢复。我们推测，荧光素酶缺陷的共生菌群氧消耗量下降，是导致lux突变体在发光器官隐窝中适合度降低的核心原因。本研究首次证实，发光能力对于细菌与其动物宿主间的正常细胞互作不可或缺，同时首次鉴定出费氏弧菌中可调控宿主正常组织发育的基因。