Antimicrobial Activity of Tea and Agarwood Leaf Extracts Against Multidrug-Resistant Microbes

Abstract

Emerging multidrug-resistant (MDR) strains are the main challenges to the progression of new drug discovery. To diminish infectious disease-causing pathogens, new antibiotics are required while the drying pipeline of potent antibiotics is adding to the severity. Plant secondary metabolites or phytochemicals including alkaloids, phenols, flavonoids, and terpenes have successfully demonstrated their inhibitory potential against the drug-resistant pathogens. In quest of potential phytochemicals, we selected tea (Camellia sinensis) and agarwood (Aquilaria malaccensis) leaves for antimicrobial activity. Fresh tea leaves were collected in three varieties, namely, BT-6, BT-7, and BT-8, including green tea (nonfermented tea), black tea (fully fermented tea), and agarwood leaves collected from Sylhet region of Bangladesh. This study is aimed at analyzing the phytochemical constituency and antimicrobial activity of tea and agarwood leaf extracts and analyzing if there is a combined effect or synergistic activity against multidrug-resistant pathogens. The antimicrobial activity of tea and agarwood leaf extracts was analyzed against MDR pathogenic bacteria and fungus. Qualitative and quantitative phytochemical constituency profiling of these six leaf extracts was evaluated, and preliminary screening exhibited that most of the leaves contained diverse groups of metabolites (alkaloids, tannin, flavonoids, glycosides, saponins, etc.). The highest amounts of TPC (total phenolic content) (110.16 ± 0.48 μg/mg) were found in BT-7 in ethanol extracts, and BT-8 in methanol extracts possessed the highest (128.1 ± 0.43 μg/mg) TFC (total flavonoid content). Notably, green tea showed remarkable results in TPC and TFC. In antioxidant scavenging activity, BT-7 and green tea showed significant IC₅₀ values which were 13.23 and 20.75 mg/mL, respectively. In antimicrobial assays, both 50 μL of each tea and agarwood leaf extract antimicrobial activities were examined against 50 μL of each bacterial and fungal culture. In synergistic activity, 50 μL of each type of leaf extracts was poured over the commercial antibiotics to evaluate their synergism, additive, or antagonism activity against the multidrug-resistant pathogens. In the antimicrobial activity test, green tea showed a maximum diameter (22.0 ± 1.1 mm) zone of inhibition against Klebsiella pneumoniae whereas BT-8 showed 22.0 ± 2.5 mm against Pseudomonas aeruginosa. Indeed, fresh tea BT-6 and BT-7 both showed remarkable zone of inhibition against the selected microbes including Gram-negative and Gram-positive bacteria. Besides, leaf extract also showed antimicrobial activity against pathogenic fungus Mucor circinelloides. Aiming to increase antibiotic resistance efficacy, synergistic activities were evaluated among leaf extracts and antibiotics against the selected pathogens where synergism, antagonism, and additive results were noted. Combination of BT-8 extracts with antibiotics (ceftiofur) showed the highest synergism nearly 36 mm of the zone of inhibition against Escherichia coli. Additionally, green tea with gentamicin and erythromycin also showed remarkable synergism 35 and 33 mm against Mucor circinelloides and E. coli, respectively. Tea and agarwood leaves grown in Bangladesh possess high antioxidant activity, promising antibacterial and antifungal activity, thus might provide a potential source for drug discovery.

Keywords: agarwood leaves; antimicrobial activity; phytochemicals; synergistic activity; tea.

Figure 1

Graphical representation of TPC (total phenolic content) (mean ± SD) of six leaf extracts: BT-8, BT-7, BT-6, BT, GT, and agarwood leaves (AM) (Aquilaria malaccensis).

Figure 2

Graphical representation of TFC (total flavonoid content) (mean ± SD) of six leaf extracts BT-8, BT-7, BT-6, BT, GT, and agarwood leaves (AM) (Aquilaria malaccensis).

Figure 3

IC₅₀ values of tea and agarwood leaf extracts.

Figure 4

Antimicrobial activity of leaves (BT-7, BT-8, BT-6, black tea, green tea, and agarwood) in methanol, ethanol, and chloroform extracts against Salmonella spp., Pseudomonas aeruginosa, Klebsiella pneumoniae, Mucor circinelloides, E. coli, and Staphylococcus aureus.