ECN菌的益生机制：ECN对肠道菌群、肠上皮屏障和肿瘤的作用

大量体外实验证明ECN具有抗菌作用（图1）。ECN可调节IBD[25]引起的肠道菌群紊乱。IBD常伴有肠道菌群紊乱，包括链霉菌(Streptomyces)减少和变形杆菌(Proteus bacteria)增加[1,2]。ECN通过F1绒毛和H1鞭毛附着在肠上皮细胞(IECs)上。同时，局部持续分泌抗生素Journal Pre-proof 5(如细菌素MccM和MccH47)拮抗沙门氏菌[3]。此外，ECN能够产生六种铁载体(即儿茶酚肠霉素及螯合物、肟酸异辛催产素、混合铁载体Y ersenin、ChuA蛋白和EfeU蛋白)与其他细菌竞争铁元素。ECN还能抑制致病菌(如肠出血性大肠杆菌)的定植和毒素(如产志贺毒素大肠杆菌)的产生，从而维持肠道菌群的稳定[4,5]。

ECN表达的菌毛和鞭毛直接刺激肠上皮细胞(IECs)产生人β-防御素[6]，人β-防御素可直接杀灭细菌，调节免疫[7]。IBD患者黏膜屏障中β-防御素水平常异常[8]。ECN在一定程度上避免了粘膜细菌附着和侵入引起的炎症。此外，已有研究证明ECN通过影响组织中miRNA的表达来调节炎症。在2,4-二硝基苯磺酸钠肠炎小鼠中，ECN可降低肠道组织中miRNA-155和miRNA-223的表达，且这两个指标与促炎因子IL-1β和TNFα呈正相关。同时，ECN上调miRNA-143，具有抑制先天免疫的作用。ECN可以通过下调基质金属蛋白酶-2和TNF-α的表达来恢复miRNA-143和miRNA-150的表达[9]。

此外，IBD患者肠上皮屏障被破坏，原因是IECs内紧密连接(tight junction, TJ)受损，IECs间渗透性增加，病原体或物质容易进入体循环。小鼠实验研究表明，ECN可促进IECs中TJ蛋白Zonula occludens-1的表达[27,28]。此外研究表明，ECN通过上调TLR-4的表达，抑制RHOA/ROCK2/MLC信号通路的激活(激活该通路会引起TJ蛋白分布的改变和屏障功能的破坏)，从而修复肠上皮屏障的破坏[10,11]。

全身给药后，ECN在动物模型中定植厌氧肿瘤组织，使其成为治疗肿瘤的一种有前途的益生菌。ECN在自身免疫环境中不破坏或诱导自体外周T细胞耐受[12]。实验表明，ECN静脉注射后可在小鼠骨肉瘤细胞(约109CFU/g肿瘤组织)内定植，定殖时间长达8d。体外实验表明，ECN和热灭活的ECN均可通过PI3K/PTEN/AKT信号通路诱导结肠癌HT-29人结肠癌细胞凋亡，从而通过上调PTEN和bcl2相关蛋白(Bax)，下调AKT而导致结肠癌细胞发生凋亡。另一项研究表明，ECN上清液可有效降低Caco-2癌细胞的活力，并显著降低caspase3/7的活性，以减轻5-氟尿嘧啶(5-Fu)化疗药物对IEC-6细胞的损伤[13]。

ECN抑制癌症的确切机制尚不清楚，还需要进一步的研究。

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