The rapid emergence of resistant Gram-negative bacteria and the limited discovery of novel antibiotic is a global healthcare challenge. Many medicinal plants with potent bioactivities have been developed for the treatment of bacterial infections. Aquilaria malaccensis exhibits wide applications from perfumes and aromatic foods ingredients and great potential in medicines. In this study, crude leaf extract of A. malaccensis was evaluated for its antibacterial activity against several pathogenic Gram-negative bacteria. The leaves were processed and extracted by Soxhlet method using ethanol as the solvent. The antibacterial activity of the crude extract was tested by disc diffusion method, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against Acinetobacter baumannii (ATCC 19606), Klebsiella pneumoniae (ATCC 10031 and ATCC 700603) and Escherichia coli (ATCC 1129). Using the optimized method, the Soxhlet extract produced a yield of 178.41 mg/g. Treatment of the extract at 200 mg/mL displayed the largest inhibition zones of 14.0 mm and 9.7 mm against A. baumannii and K. pneumoniae ATCC 10031, respectively. In contrast, against E. coli and K. pneumoniae ATCC 700603, smaller zones of inhibitions of 3.3 mm were demonstrated. The MIC values of the extract were 32 mg/mL against A. baumannii and K. pneumoniae ATCC 10031 and 64 mg/mL against E. coli and K. pneumoniae ATCC 700603. The MBC values of the extract were consistent with the MIC values for all the bacteria investigated. Overall, this study was the first to show antibacterial activity of A. malaccensis leaves extract particularly against A. baumannii and K. pneumoniae and potentially develop for the treatment of resistant bacteria.

Aims:Polymyxins are an important last-line treatment for infections caused by multidrug-resistant Gram-negative bacteria. Nonetheless, the emergence of polymyxin-resistance and the limiting of polymyxin monotherapy urgently demands its optimisation. Aquilaria malaccensis (Agarwood) has been widely used as traditional medicine. Many parts of the plant including leaves exhibit a considerable in vitro antibacterial activity against microbial pathogens. Exploiting A. malaccensis in combination with polymyxins provides a novel strategy in fighting antimicrobial resistance. The objective of this study was to evaluate the combination effects of A. malaccensis extract with polymyxins against Acinetobacter baumannii and Klebsiella pneumoniae. Methodology and results: In vitro time-kill studies and GC-MS analysis were performed to evaluate the bacterial killing of polymyxin B and extract combination and analyse chemical compounds of the extract, respectively. The combination of polymyxin B (1 mg/L) and A. malaccensis extract (32 mg/mL and 64 mg/mL) treatments exhibited enhanced bacterial killing compared to polymyxin B alone at 4 h and 24 h. Combination treatments also inhibited the bacterial growth of both A. baumannii and K. pneumoniae observed throughout the 24 h. More than sixty compounds including phytol, 9,12octadecadienal, fatty acid, alkanes and terpenoids were putatively identified as the compounds that likely contributed to the antibacterial activity. Conclusion, significance and impact of study:This study was the first to report the potential application of A. malaccensis extract in combination with polymyxin B in treatment against A. baumannii and K. pneumoniae and can be further investigated and optimized for the treatment of bacterial infectious diseases.