Synergistic and antagonistic interactions of triclosan with various antibiotics in bacteria

Triclosan (TCS), a well-studied antimicrobial compound and an environmental pollutant, is present in many household products. A systematic survey of TCS-antibiotic-bacteria interactions is lacking. We wish to understand the origin of such interactions by testing 16 phylogenetically well-characterized bacteria for their sensitivities to 6 different classes of antibiotics with or without the presence of TCS. Our results show that TCS interacts synergistically with some antibiotics against some Bacilli species. TCS could also interact antagonistically with other antibiotics against certain bacteria, including pathogens such as Pseudomonas aeruginosa and Stenotrophomonas maltophilia. Antagonism between drugs often coincided with the concomitant enhanced removal of Ethidium bromide (EtBr) from the cells. Enterococcus faecalis shows a unique response to TCS. High levels of TCS inhibits E. faecalis. Cells survive at lower TCS concentrations, and these cells can remove EtBr more readily than unexposed cells. At even lower TCS concentration, cell-growth is inhibited again, causing the culture to exhibit a unique extra inhibition zone around the TCS-disk. The TCS-antibiotic-bacteria interaction profiles of some bacteria do not follow their bacterial phylogenetic relations. This suggests that such interactions may be related to horizontal gene transfer among different bacteria.