**Background:** Lung adenocarcinoma (LADC) is the most prevalent subtype of lung cancer, accounting for approximately 40% of all lung cancer cases. Previous research has suggested that gut and lung microbiota dysbiosis may be associated with lung cancer development, and the 'gut-lung axis' has been proposed to explain crosstalk between these sites. However, the specific bacterial species and metabolic pathways linking gut and lung microbiomes to LADC remain poorly understood.
**Methods:** This study enrolled 43 patients with newly diagnosed LADC (who had not received prior anticancer therapy or antibiotics within one month) and 64 healthy controls (family members of patients). Fecal samples were collected from all 107 participants, and paired BALF samples were obtained from 42 of the 43 LADC patients. The V3-V4 regions of the 16S rRNA gene were sequenced using the Illumina NovaSeq 6000 platform, and shotgun metagenomic sequencing was performed on fecal samples. Taxonomic classification was performed using QIIME2 with the SILVA database, and metagenomic reads were analyzed using MetaPhlAn3. Functional pathway analysis was conducted using KEGG database annotations, and virulence factors were identified using the VFDB database. Phylogenetic analysis of shared genera between paired gut and lung samples was performed using FastTree 2.1.10 and NCBI BLAST.
**Key Results:** A total of 20,940,816 raw reads were obtained from 107 gut samples (average 195,709 reads per sample), and 6,161,611 raw reads from 42 BALF samples (average 146,705 reads per sample). Alpha-diversity (Shannon index, evenness index, observed features) was significantly reduced in LADC patients compared to healthy controls. Beta-diversity analysis showed clear separation between groups (unweighted UniFrac: R = 0.1152, p = 0.001; Bray-Curtis: R = 0.1104, p = 0.002). At the genus level, Coprococcus, Dorea, Lachnospiraceae UCG-001, Ruminococcus torques group, Eubacterium hallii group, and Dialister were enriched in healthy controls, while Prevotella, Enterococcus, Flavonifractor, Clostridium innocuum group, and Eggerthella were enriched in LADC patients. At the species level, Bacteroides plebeius, Dialister sp. CAG-357, Eubacterium sp. CAG-38, Roseburia intestinalis, and Ruminococcus torques were enriched in healthy controls, while Bacteroides thetaiotaomicron, Fusobacterium mortiferum, Flavonifractor plautii, Enterococcus gallinarum, and Eggerthella lenta were enriched in LADC patients. Flavonifractor, Eggerthella, and Clostridium innocuum group increased with cancer progression and metastasis. A total of 94 genera were shared between fecal and BALF samples. Six shared species were identified across paired samples: Streptococcus salivarius (5 patients), Haemophilus parainfluenzae (10 patients), Fusobacterium mortiferum (1 patient), Bacteroides fragilis (1 patient), Bacteroides uniformis (1 patient), and Bacteroides stercoris (1 patient). Metagenomic analysis identified 18 differentially enriched KEGG pathway modules, with leucine degradation, propanoyl-CoA catabolism, and fatty acid biosynthesis pathways enriched in LADC patients. Thirteen virulence factors were more abundant in LADC patients, including 7 lipopolysaccharide biosynthesis genes.
**Clinical Implications:** This study identifies specific gut bacterial species associated with LADC progression and demonstrates that Haemophilus parainfluenzae is a shared species between gut and lung microbiomes in LADC patients, suggesting it may be a key mediator in the gut-lung axis. The enrichment of metabolic pathways related to leucine degradation, propanoyl-CoA catabolism, and fatty acid biosynthesis in LADC patients points to potential therapeutic targets, as inhibition of acetyl-CoA carboxylase has previously been shown to limit tumor growth in NSCLC mouse models. The increased abundance of virulence factors, particularly LPS biosynthesis genes, suggests that gut microbiota may influence lung cancer through immune modulation and inflammatory pathways. These findings support the need for larger multi-center studies and experimental validation to establish causal relationships between gut microbiota and LADC development.