Selective Growth-Inhibiting Effects of Compounds Identified in Tabebuia impetiginosa Inner Bark on Human Intestinal Bacteria

The growth-inhibiting activity of anthraquinone-2-carboxylic acid and lapachol identified in the inner bark of taheebo, Tabebuia impetiginosa, toward 10 human intestinal bacteria was evaluated by using a paper disk diffusion bioassay and compared to those of seven lapachol congeners (1,4-naphthoquinone, naphthazarin, menadione, lawsone, plumbagin, juglone, and dichlone) as well as two commercially available antibiotics, chloramphenicol and tetracycline. Anthraquinone-2-carboxylic acid exhibited very strong growth inhibition of Clostridium paraputrificum at 1 μg/disk while 100 μg/disk of lapachol was needed for moderate growth inhibition of the same organism. These two isolates exhibited weak inhibition of Clostridium perfringens and Escherichia coli at 100 μg/disk while no adverse effects were observed on the growth of Bifidobacterium adolescentis, Bifidobacterium bifidum, Bifidobacterium infantis, Lactobacillus acidophilus, and Lactobacillus casei at 1000 μg/disk. Structure−activity relationships indicate that a methyl group in the C-2 position of 1,4-naphthoquinone derivatives might play an important role in antibacterial activity. Keywords: Natural antibacterial agent; taheebo; intestinal bacteria; anthraquinone-2-carboxylic acid; lapachol; 1,4-naphthoquinones; structure−activity relationship