Relationship Between Chemical Structures of Phytochemicals, Synthetic Phytochemical Analogs, and Antibiotics and Their In Vitro Growth-Inhibitory Effects Against Colorectal Cancer-Causing Bacteria

Colorectal cancer (CRC) has been increasingly associated with gut microbiota dysbiosis and the presence of specific bacterial pathogens. This study evaluated the in vitro growth-inhibitory activity of 18 biologically active compounds, including phytochemicals, synthetic analogs, and clinically used antibiotics, against CRC-associated bacterial strains. Minimum inhibitory concentrations (MICs) were determined using the broth microdilution method and analyzed in relation to chemical structure. Conventional antibiotics, particularly tetracycline and ciprofloxacin, exhibited the strongest antibacterial activity. Among non-antibiotic compounds, nitroxoline and carbadox showed moderate activity, whereas quaternary benzylisoquinoline-derived alkaloids and polyphenols were less effective. Structure–activity relationship analysis suggested that aromatic heterocyclic scaffolds, electron-withdrawing substituents, and metal-chelating groups contribute to antibacterial potency. We obtained novel MIC data for several compounds, including ferron and oxyquinoline, against underexplored CRC-associated bacterial strains. These findings expand current knowledge of the antibacterial activity of structurally diverse compounds against CRC-associated bacteria and provide a basis for future studies on microbiota-targeted antimicrobial strategies.