Absence of bactericidal activity of orally administered wood creosote on human intestinal bacterial flora

[Abstract] Wood creosote is a mixture of simple phenolic compounds that has long been used for over a century as an antidiarrheal medicine. While there are many pharmacological studies that explain its antidiarrheal effects, it has long been a matter of controversy whether the antidiarrheal activity of wood creosote is attributable to its putative bactericidal effect on the human intestinal bacterial flora. The objective of our study was to investigate the putative bactericidal effect of wood creosote in the human intestine when given in an ordinary therapeutic dose. To this end, we used an in vitro test to measure the minimal inhibitory concentration of wood creosote for various bacteria from the human gut. In addition, we also quantified the copy numbers of bacterial 16S ribosomal DNA in the feces of healthy human participants after administration of a therapeutic dose of wood creosote. We determined the minimal inhibitory concentration of wood creosote to be >128 μg/ml, a level far greater than that of commonly used antibacterial agents. Copy numbers of bacterial 16S ribosomal DNA in human feces after the administration of a therapeutic dose of wood creosote did not change significantly ( p > 0.05) from that before dosing. Taken together, we conclude that wood creosote given at an ordinary therapeutic dose has no significant antibacterial effect in the human lower intestine where most of the intestinal bacterial flora resides. Thus, the antidiarrheal properties of wood creosote are not attributable to its effect on the intestinal bacterial flora, but rather to its other effects on the intestine.