MRSA exhibited resistance to oxacillin while MSSA showed susceptibility. Most MSSA showed susceptibility to ampicillin while MRSA showed resistance. All MRSA showed resistance to ciprofloxacin while MSSA showed susceptibility. For the mecA gene determination all MRSA isolates showed mecA gene while MSSA did not.
The aqueous crude extracts of natural products and fresh juice of Capsicum flutescens L. and Piper nigerum were not found to be active against all bacteria tested, i.e. MSSA clinical isolates, MRSA clinical isolates, MSSA (ATCC25923), and MRSA (ATCC43300). Citrus hystrix (bergamot oil) showed the largest inhibition zone against all bacteria tested (Table 1). As a result, bergamot oil was marked as the chosen natural product extract for the study of synergism with antibiotics.
The study of synergistic effects between bergamot oil and antibiotics tested against MRSA (ATCC43300) and MSSA (ATCC25923) showed that the inhibition zone of the combination of bergamot oil and tetracycline against the ATCC strains of MRSA and MSSA was greater than that of the bergamot or tetracycline alone (Table 2). Therefore, tetracycline was marked as the chosenantibiotic for the synergistic study. In addition, the combination of bergamot oil and tetracycline reduced the MIC and MBC of tetracycline against the ATCC strains of MRSA and MSSA (Table 3). Kinetics of kill study showed that the mixture of bergamot oil and tetracycline was superior to bergamot oil or tetracycline alone in the rate at which it killed both MRSA and MSSA as shown in Table 4. The mixture of bergamot oil and tetracycline killed MRSA and MSSA at least 3 logs in 30 minutes whereas tetracycline and bergamot oil separately took more than 30 minutes to achieve the same kill.