Anti-Acinetobacter baumannii activity of selected phytochemicals alone, in binary combinations and in combinations with conventional antibiotics

Abstract

Acinetobacter baumannii is one of the emerging multidrug- and pandrug-resistant pathogens. The aim of the study was to determine anti-A. baumannii activity of selected terpenes, terpenoids and phenylpropanoids alone, in binary combinations, and in combinations with conventional antibiotics using microdilution-checkerboard and time-kill curve method. The most effective were terpenoids carvacrol (7.0–28.0 μg mL^–1) and thymol (22.0–76.0 μg mL^–1), as well as phenylpropanoid compound eugenol (90.5–304.0 μg mL^–1), with the active concentrations were comparable to antibiotics. The binary combinations showed additive or indifferent effects. The combination gentamicin-carvacrol was synergistic only against reference strain (FICI = 0.50), while other combinations were additive. The best bacteriostatic activity showed carvacrol, thymol or eugenol in combination with ciprofloxacin (FICI range 0.11–0.50) against both the reference and multidrug-resistant strains. The synergistic effect was further confirmed by time kill curve method and obtained after only 15 h. The results indicate a new possible therapeutic strategy against multiple resistant A. baumannii.

Graphical Abstract

Keywords:

• Acinetobacter baumannii
• thymol
• carvacrol
• eugenol
• gentamicin
• ciprofloxacin
• synergism

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia [grant no. 451-03-68/2020-14/ 200125].