Background: Formulations employing synergistic combinations of antibiotics
with Essential Oils (EOs) could help preserve the antibiotic repertoire by improving their activity
against resistant bacteria.
Objective: This study was aimed to screen the antibiotics oxacillin and ciprofloxacin for
synergistic interactions with cumin, oregano and rosewood EOs and the EO components
cuminaldehyde, carvacrol and linalool against Escherichia coli, Pseudomonas aeruginosa
and Staphylococcus aureus (antibiotic sensitive and resistant isolates). This will provide information
on formulations with synergistic combinations of EOs and antibiotics that might
resensitise antibiotic resistant bacteria.
Methods: Antimicrobial interactions between double and triple combinations of EOs, EO
components and antibiotics were determined using the checkerboard method. The most active
triple combinations were then assessed by a time-kill assay.
Results: Two synergistic EO-antibiotic combinations and eight additive EO-antibiotic combinations
reduced the antibiotic minimum inhibitory concentration below clinical sensitivity
breakpoints according to the checkerboard method. However, all the tested combinations
were additive according to the time-kill assay; while the combinations completely killed
S. aureus, E. coli and P. aeruginosa cells in 2 h. At least one EO compound from the combination
alone completely killed the cells of test species.
Conclusion: Positive interactions support the use of EOs or EO components to enhance antibiotic
efficacy against antibiotic resistant bacteria. The EO-antibiotic combinations tested
by the time kill assay were indifferent; therefore, the observed antimicrobial activity did not
arise from synergistic mechanisms as indicated by the checkerboard method. Investigation of
other synergistic combinations identified by the checkerboard method could reveal more