In recent times, methicillin-resistant Staphylococcus aureus infections caused serious health problems all over the world [1]. Isoflavones, a subclass of isoflavonoids naturally occurring in legume-based human diet, belong among a number of possible candidates of leading compounds for development of new effective anti-staphylococcal agents [2]. Demethyltexasin (6,7,4'-trihydroxyisoflavone) is a metabolite of daidzein naturally occurring in soy-based food products, such as doenjang and tempeh [3]. This compound has recently been reported to produce significant anticancer activity [4], however, a little is known about its other biological properties. Therefore, we decided to examine synergistic effect of demethyltexasin in combinations with oxacillin against five strains of S. aureus, including three resistant to methicillin.
The minimum inhibitory concentrations were determined by the broth microdilution method [5], whereas the combinatory effect was evaluated according to the sum of fractional inhibitory concentration (ΣFIC) indices by the checkerboard method [6]. As a result, an anti-staphylococcal synergistic effect was observed against three out of five strains tested with ΣFIC values ranging from 0.250 to 0.424. Since all of these three strains were methicillin resistant, our results suggest that demethyltexasin selectively affects specific mechanisms responsible for methicillin resistance.
According to our best knowledge, this is the first report on synergistic antimicrobial effect of demethyltexasin. Our results suggest this compound as an attractive candidate for the development of new synergistically acting anti-staphylococcal drugs.