Evaluation of selected natural sesquiterpenes as sensitizing agents of ß-lactam-resistant bacterial strains.

ABSTRACT

AIMS: To evaluate the capacity of fourteen sesquiterpenes to enhance the action of known antibiotics against two ß-lactam resistant strains, and to determine a possible mechanism of antibiotic sensitization by assessing their ability to inhibit a ß-lactamase enzyme. METHODS AND RESULTS: The broth microdilution method was used to determine the minimum inhibitory concentration (MIC) of ß-lactams cefuroxime (CEFM) and cefepime (CPM) against Staphylococcus aureus 23MR and Escherichia coli 82MR strains in the absence and presence of subinhibitory concentrations of fourteen natural sesquiterpenes. (1R,4R)-4H-1,2,3,4-tetrahydro-1-hydroxycadalen-15-oic acid (5), xerantolide (8), estafiatin (11), and ambrosin (12) exhibited the best sensitizing effects in both strains. These compounds were able to reduce the MIC of CEFM by 2-fold (from 15.0 to 7.5 µg/mL) and CPM by 15-fold (from 0.9 to 0.06 µg/mL) in S. aureus 23MR. For E. coli 82MR, the MIC of CEFM was reduced up to 8-fold (from 120.0 to 15.0 µg/mL). In this strain, the activity of 8 and 11 surpassed that of clavulanic acid (positive reference), which reduced the MIC of CEFM from 120.0 to 60.0 µg/mL. To elucidate a possible mechanism of antibiotic sensitization, molecular docking studies were conducted with ß-lactamases. These studies revealed an affinity with the enzymes (energies > -4.93 kcal/mol) by the formation of hydrogen bonds with certain conserved amino acid residues within the active sites. However, the in vitro results indicated only marginal inhibition, with percentages <50%. CONCLUSIONS: The bioevaluations indicate that nine of fourteen sesquiterpenes enhance the action of CEFM and CPM against the ß-lactam resistant strains, and these compounds displayed moderate activity as inhibitors of ß-lactamase.