Antibacterial activities of various flavonoids have been reported previously, but mechanism(s) of their action on bacterial cells remain(s) largely unknown. Here, we investigated effects of genistein, an isoflavone, and representatives of other flavonoids: daidzein (another isoflavone), apigenin (a flavone), naringenin (a flavanone) and kaempferol (a flavonol), on commonly used laboratory strains of model bacterial species: Escherichia coli, Vibrio harveyi and Bacillus subtilis. We found that E. coli was resistant to all tested flavonoids at concentrations up to 0.1 mM, while high sensitivity of V. harveyi to most of them (except daidzein, which exhibited significantly less pronounced effect) was observed. Effects of the flavonoids on B. subtilis were relatively intermediate to the two extremes, i.e., E. coli and V. harveyi. Action of genistein on bacterial cells was investigated in more detail to indicate changed cell morphology (formation of filamentous cells) of V. harveyi and drastic inhibition of global synthesis of DNA and RNA as shortly as 15 min after addition of this isoflavone to a bacterial culture to a final concentration of 0.1 mM. Protein synthesis inhibition was also apparent, but delayed. Both cell morphology and synthesis of nucleic acids and proteins were unaffected in E. coli cultures under analogous conditions. Studies on cell survival suggest that genistein is a bacteriostatic agent rather than a bactericidal compound.

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Previously performed experiments showed that methylxanthines, especially caffeine, may protect cells against cytostatic or cytotoxic effects of several aromatic compounds. One of the proposed mechanisms of this protection is based on stacking interactions between pi electron systems of polycyclic aromatic molecules. In this work, we demonstrate that caffeine and other methylxanthines--pentoxifylline and theophylline--significantly decrease mutagenicity of the anticancer aromatic drugs daunomycin, doxorubicin and mitoxantrone. The spectrophotometric titration of these aromatic compounds by methylxanthines indicated formation of mixed aggregates. The concentrations of free active forms of the drugs decreased when the concentrations of methylxanthines increased in the mixture. Therefore, likely methylxanthines may play a role of scavengers of the free active forms of daunomycin, doxorubicin and mitoxantrone.

Antibacterial activities of various flavonoids, a group of natural plant substances, have been reported previously, however, there are contradictory data, published by various authors, regarding sensitivity of particular bacterial species to these compounds. These problems arose apparently because of using different methods by various researchers. Here we tested sensitivity of several bacterial species (Gram-positive: Bacillus subtilis, Micrococcus luteus, Sarcina sp. and Staphylococcus aureus; and Gram-negative: Citrobacter freundii, Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, Pseudomonas aeruginosa, Salmonella enterica, Serratia marcescens and Vibrio harveyi) to various flavonoids: genistein and daidzein (isoflavones), apigenin (a flavone), naringenin (a flavanone) and kaempferol (a flavonol) by measurement of generation times of bacteria in liquid cultures. The presented results indicate that this simple method is adequate for unambiguous assessment of sensitivity of bacterial strains to flavonoids.