Evaluation of Catha edulis and Thymus serrulatus Extracts for their Antibacterial, Antioxidant, Antibiofilm and anti-Quorum-Sensing Activities on Selected Pathogenic Bacterial Species.

Abstract

Antimicrobial resistance is progressively increasing at an alarming rate which make infections untreatable by conventional antibiotics over time. Natural products from plants are potential sources of possible solutions to combat this challenge. In this study, we have evaluated two plants, Catha edulis and Thymus serrulatus for phytochemicals, antibacterial, antioxidant, antibiofilm and anti-Quorum sensing potential. The GC-MS analysis of the Dichloromethane extracts identified phytol at 60.3%, Benzyl_carbazate at 20% and squalene at 4.4%; the ethyl acetate) extract contained (24R)-Stigmast-5-en-3.beta at 33%, β-Amyrin at 28%, squalene, α Tocopheryl acetate at 4% and the essential oil had 1,4-Cyclohexadiene, 1-methyl-4-(1- methylethyl)- at 35%, p-Cymene at 2% and Carvacrol methyl ether at 1%, respectively as the major compounds. Antibacterial activity was done by agar disk diffusion assay. All the extracts tested were able to inhibit bacterial growth: ethyl acetate (55-77%), Dichloromethane (53- 73%), methanol (51-59) and essential oil (47-59%), respectively. Antioxidant activity was done by DPPH assay. The extracts exhibited high antioxidant property (78.3-89.18% radical scavenging activity). The minimum inhibitory concentration for the extracts of C. edulis were in the range (500-1000 µg/ml) and essential oil of T. serrulatus was 0.31-0.62 µl/ml, respectively. The minimum bactericidal concentration values were 2 mg/ml and 1.25-2.5 µl/ml for the same extracts. The Tolerance test values were in the bactericidal range of 2-4 for different bacteria except for P. aeruginosa that recorded a value of 8 and indicated bacteriostatic action. Antibiofilm activity was done by crystal violet staining assay. The methanol extract was found to have a dose-dependent inhibitory effect on biofilm formation in tested gram-positive bacteria, however, it did not exhibit a strong inhibition against biofilm formation in tested gram-negative that remained relatively constant at ~40% even at higher concentrations. The Dichloromethane extract demonstrated stronger inhibitory activity against gram-positive bacteria compared with gram-negative bacteria. The ethyl acetate extract and essential oil were able to effectively inhibit biofilm formation in gram-positive as well as gram-negative bacteria. All the C. edulis extracts exhibited inhibitory effects on pyocyanin production in P. aeruginosa (>44% at the highest concentration). The five major compounds generated during GC-MS analysis were evaluated for binding potential against four targets that are involved in Quorum sensing (LasI, LuxR, LsrR and AgrC) by molecular docking. All the targets bound the major phytocompounds with high energy and demonstrated strong binding indicating that these compounds may have the potential to effectively inhibit bacterial growth and biofilm formation via anti-Quorum sensing activity. Therefore, Catha edulis and Thymus serrulatus may be regarded as potential sources of antibacterial as well as precursors of individual antibacterial compounds with a potential to be investigated and developed further.