Antibacterial activity and synergistic effects between Eucalyptus globulus leaf residues (essential oils and extracts) and antibiotics against several isolates of respiratory tract infections (Pseudomonas aeruginosa)

• Antibacterial activity of Eucalyptus globulus residues was tested against Pseudomonas aeruginosa.
• 16 isolates were tested with leaf extracts and essential oils revealing susceptibility.
• Synergistic tests of these residues with antibiotics were performed for the first time.
• The combination of antibiotic with extract or essential oil increased the antibacterial activity.
• These tests showed an addictive effect in 55% of the cases.

This is a novel study of phenolic content and synergistic effects between extracts and Eucalyptus globulus leaves essential oil and antibiotics against several isolates from respiratory tract infections (Pseudomonas aeruginosa). The extraction yield obtained with polar solvents was substantially higher, with the maximum value found for methanol 70%, followed by methanol 100% and water. Of the 16 isolates tested, only 4 (MJH 4, MJH 15, MJH 21 and MJH 40) revealed susceptibly to the extracts tested. The MJH 4 revealed the highest susceptibility of the 5 extracts tested (water, 70% methanol, 100% methanol, 100% acetone and 100% dichloromethane). The isolates tested showed the highest susceptibility to the extracts with higher phenolic compounds content, i.e. the methanolics. The highest inhibition halo value obtained was observed for the oil B against the isolate MJH 4 (19.3 mm), while the minimum value was obtained against the isolate MJH 207 (7.7 mm) with oil A. The MIC (minimum inhibitory concentration) value for the extracts ranged between 625.0 μg/mL and 2500.0 μg/mL, while it was also observed that the essential oil B was more efficient than oil A. The combination between antibiotic and extracts or essential oils demonstrated an additive effect in 55% of the cases, meaning that the combined use of antibiotic and the compound tested (extract or essential oil) revealed an increase of antibacterial activity. The use of these compounds (extracts and oils) of E. globulus may represent an important source of bioactive compounds and an alternative for the treatment of respiratory infectious diseases caused by P. aeruginosa.