Evaluation of the mutagenicity of sesquiterpenic compounds and their influence on the susceptibility towards antibiotics of two clinically relevant bacterial strains

Sesquiterpenic compounds are natural chemicals present in organisms from different Phylae or Divisions, which have proved to be important bioactive products, namely in potentiating the action of antibiotics. In the first step, the mutagenicity of nine sesquiterpenic compounds (hydrocarbons and alcohols) was screened in a Salmonella typhimurium his−-reversion test with strains TA98 and TA100, in the presence or absence of in vitro metabolic activation. Under the test conditions, none of the compounds showed mutagenicity up to a concentration of 222 μg/plate. trans-Farnesol, nerolidol, and α-bisabolol displayed cytotoxicity when tested at concentrations ranging from 14 to 222 μg/plate. Then, the combined effect of antibiotic–sesquiterpenic compounds was evaluated on two clinically relevant pathogens, Escherichia coli and Staphylococcus aureus, with well-defined resistance-sensitive profiles. The agar-disc diffusion assay revealed that all the combinations of antibiotic–sesquiterpenic compounds increased the antibacterial activity of the antibiotics tested against S. aureus. For E. coli, an antagonistic effect was observed for various combinations on the growth of this bacterium.