In Vitro Antibacterial Potential of Pinus nigra-Thymus serpyllum Essential Oil and Antibiotic Combinations

Synergistic interactions have been recognized and used for centuries, with various healing traditions using herbal combination treatments to enhance their effects. In the recent years, essential oil combinations were used effectively. Pinus nigra JF Arnold, and Thymus serpyllum L. essential oils, which are mainly used in colds and respiratory diseases, were examined for the efficacy, individually and in combination against 12 different human pathogens. The antimicrobial effects were evaluated using the in vitro microdilution method and the checkerboard assay to assess potential synergistic interactions. Herein comparative inhibitions by T. serpyllum and P. nigra oils with the combination of ciprofloxacin, amoxicillin, and tetracycline antibiotics were tested, respectively. The major components of the essential oils were also evaluated in combination. The quality of the commercial essential oils were analyzed by GC/MS and GC/FID for quality confirmation. The main components for P. nigra were α-pinene (73.8%), trans-verbenol (4.4%), and α-pinene oxide (2%), respectively, whereas for T. serpyllum, the primary components were geraniol (19.3%), p-cymene (12.9%), α-thujone (6.6%), linalool (6.4%), and carvacrol (5.7%), respectively. The microbiological evaluations resulted in a synergistic effect of T. serpyllum + P. nigra oil combination against Enterococcus faecalis (FICI: 0.187), and Staphylococcus epidermidis (FICI: 0.375). The combination of P. nigra and amoxicillin also resulted in a synergistic effect against the Streptococcus mitis (FICI: 0.185) strain. In addition, the combination of the main components of the essential oils were evaluated against all microorganisms in the panel and resulted in independent activity. Using the in vitro MTT assay, the synergistic combinations resulted in no toxicity.