To investigate how intestinal bacteria affect host cytogenetic alterations in the early initiation step of colon carcinogenesis, we conducted a comet assay and micronucleus (MN) test using germ-free (GF) and conventionalized (Cvd) rats after a single subcutaneous injection of the carcinogen, 1,2-dimethylhydrazine dihydrochloride (DMH). DNA damage was also determined in the liver in comet assays, as DMH is metabolized and activated in this organ. The time-response patterns of DNA damage in the liver and colon were similar in both rats, and maximum values were observed at 3 h after the treatment. In contrast, the maximum frequency of micronucleated (MNed) colonic cells was markedly higher in the Cvd rats than in the GF rats and was observed after 72 h and 120 h, respectively. The frequency of MNed cells in non-treated animals was similar in the GF and Cvd rats. In addition, we determined time-responses in the incidence of apoptosis and cell proliferation indices, i.e., the numbers of BrdU-labeled cells, mitotic cells in the crypts, and crypt column heights, using histological sections of the colons in these rats. Maximal incidence of apoptosis was observed at 6 and 24 h in the Cvd and GF rats, respectively. The value in the Cvd rats tended to be higher than that in the GF rats. Cell proliferation was suppressed until 24 and 48 h in the Cvd and GF rats, respectively, and increased subsequently. The rebound response of cell proliferation was more pronounced and occurred earlier in the Cvd rats than that in the GF rats. We demonstrated that cytogenetic alterations other than DNA damage, particularly the MNed colonic cell induction by DMH, were markedly enhanced in rats with bacterial colonization in the intestine compared to those in GF rats.

中文翻译：

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1,2-二甲基肼二盐酸盐在肠道细菌定殖的大鼠结肠细胞中显着增强了微核诱导作用。

为了研究肠道细菌如何在结肠癌发生的早期阶段影响宿主细胞遗传学改变，我们在单次皮下注射致癌物后，使用无菌（GF）和常规（Cvd）大鼠进行了彗星分析和微核（MN）测试，1,2-二甲基肼二盐酸盐（DMH）。由于DMH在该器官中被代谢和激活，因此在彗星试验中还可以确定肝脏中的DNA损伤。在两只大鼠中，肝脏和结肠中DNA损伤的时间响应模式相似，并且在治疗后3 h观察到最大值。相反，Cvd大鼠中微核（MNed）结肠细胞的最大频率明显高于GF大鼠，分别在72 h和120 h后观察到。在未经治疗的动物中，MNed细胞的频率在GF和Cvd大鼠中相似。另外，我们使用这些大鼠结肠的组织学切片确定了凋亡发生率和细胞增殖指数（即BrdU标记的细胞数，隐窝中的有丝分裂细胞数和隐窝柱高）的时间响应。在Cvd和GF大鼠中分别在6和24 h观察到最大的凋亡发生率。Cvd大鼠中的值倾向于高于GF大鼠中的值。在Cvd和GF大鼠中，细胞增殖分别被抑制到24和48 h，随后增加。与GF大鼠相比，Cvd大鼠的细胞增殖反弹反应更为明显，并且发生得更早。我们证明了DNA损伤以外的细胞遗传学改变，