Mechanisms of the gut-lung axis in regulating lung metastasis of colorectal cancer and advances in traditional Chinese medicine intervention

Table 1 Key pro-metastatic bacteria and their mechanisms

Bacterium	Virulence factor	Mechanism promoting metastasis	Evidence level (additional notes)
Fusobacterium nucleatum	FadA, Fap2	E-cadherin/β-catenin, TLR4/NF-κB, exosomal miR-1246	Preclinical + human correlative; most extensively studied
Enterotoxigenic Bacteroides fragilis	BFT	E-cadherin cleavage, induction, neutrophil recruitment	Preclinical only (so far)
pks+ E. coli	Colibactin	DNA damage, genomic instability, mutational signatures	Human colorectal tumor evidence; direct lung metastasis link less clear
Peptostreptococcus anaerobius	Putative surface protein	TLR2/4 activation, ROS production	Preclinical
Parvimonas micra	Unknown	Associated with advanced stage and metastasis	Human cohort study; virulence factor unknown

TLR4: Toll-like receptor 4; NF-κB: Nuclear factor kappa B; BFT: Bacteroides fragilis toxin; E. coli: Escherichia coli; ROS: Reactive oxygen species.