Background/Objectives: Malignant ascites frequently arises in advanced cancers with peritoneal metastasis and is associated with poor outcomes. Known mechanisms include lymphatic obstruction by tumor cells, increased vascular permeability, and sodium retention via the renin–angiotensin–aldosterone system; however, the pathogenesis remains not fully understood. We investigated whether gut and bladder microbiomes correlate with malignant ascites development or progression and whether the immune microenvironment in ascitic fluid is altered. Methods: We enrolled 66 histologically confirmed cancer patients, dividing them into malignant ascites (n = 20) and non-ascites (n = 46) groups. Stool, urine, and ascitic fluid samples were analyzed using 16S rRNA next-generation sequencing. Immune cell subsets in ascitic fluid were characterized using flow cytometry. Results: In 19 of the 20 malignant ascites samples, the bacterial load was too low for reliable 16S rRNA sequencing, suggesting that malignant ascites is largely sterile. The overall gut microbiome diversity did not differ significantly by ascites status, although a trend emerged in patients with peritoneal metastasis, including the enrichment of class Clostridia and Gammaproteobacteria. Bladder microbiome analysis also showed no significant differences in ascites or metastasis status. Flow cytometry revealed reduced T-cell (CD3+, CD4+, CD8+) and NK cell (CD56+) populations compared to data from cirrhotic ascites. Conclusions: Malignant ascites exhibit minimal bacterial biomass, making comprehensive microbiome analysis challenging. Although no major global changes were noted in gut and bladder microbiomes, specific taxa were linked to peritoneal metastasis. These findings highlight an immunosuppressive ascitic environment and suggest that larger-scale or multi-omics approaches may help elucidate the role of microbiota in malignant ascites.
**背景/目的：** 恶性腹水常见于发生腹膜转移的晚期癌症患者，并与不良预后相关。已知机制包括肿瘤细胞导致的淋巴管阻塞、血管通透性增加以及肾素-血管紧张素-醛固酮系统介导的钠潴留；然而，其发病机制仍未完全阐明。本研究旨在探究肠道和膀胱微生物组是否与恶性腹水的发生或进展相关，以及腹水中的免疫微环境是否发生改变。 **方法：** 我们纳入了66例经组织学确诊的癌症患者，将其分为恶性腹水组（n=20）和无腹水组（n=46）。使用16S rRNA新一代测序技术对粪便、尿液和腹水样本进行分析。采用流式细胞术对腹水中的免疫细胞亚群进行表征。 **结果：** 在20份恶性腹水样本中，有19份的细菌载量过低，无法进行可靠的16S rRNA测序，提示恶性腹水在很大程度上是无菌的。尽管在腹膜转移患者中观察到梭菌纲和γ-变形菌纲富集的趋势，但总体肠道微生物组多样性在有无腹水状态间无显著差异。膀胱微生物组分析也未显示腹水或转移状态存在显著差异。流式细胞术分析显示，与肝硬化腹水数据相比，恶性腹水中T细胞（CD3+、CD4+、CD8+）和NK细胞（CD56+）群减少。 **结论：** 恶性腹水细菌生物量极低，使得全面的微生物组分析面临挑战。尽管未观察到肠道和膀胱微生物组发生显著的全局性变化，但特定分类群与腹膜转移相关。这些发现突显了恶性腹水的免疫抑制环境，并提示更大规模或多组学方法可能有助于阐明微生物群在恶性腹水中的作用。
肿瘤(癌症)患者之家