In vitro antidiabetic and antimicrobial properties of Ocimum species (Ocimum basilicum and Ocimum sanctum) (L.)

Abstract

Introduction In Africa, use of phytotherapy for treatment of diabetes mellitus is a common form of practice. Considering the increasing burden of non-communicable diseases in South Africa efforts are directed at simple, cost effective, non-hazardous and efficient methods to treat cancer, cardiovascular diseases and diabetes. The role of phytonanotherapy is an attractive proposition for advancing new therapies. Metal nanoparticles are a possible means for delivery of such therapies. However, this requires investigation on interactions, mechanisms and therapeutic efficacy upon co-administering ethnobotanicals with metal nanoparticles and existing drug therapy in human beings.

Aim The primary aim of the study was to test the in vitro antidiabetic and antibacterial activity of Ocimum sanctum (leaf extracts and flower extracts), Ocimum basilicum (leaf extracts and flower extracts), and a combination of the leaf extracts of both, and to observe whether any antidiabetic and antibacterial activity was enhanced in due to phyto-synthesised bimetallic gold-silver (Au-Ag) nanoparticles and silver nanoparticles.

Methods Aqueous and ethanol extracts of O. sanctum and O. basilicum leaf and flowers alone and combined (leaf + flower) were prepared using hot vs cold water extraction techniques and 60% and 70% ethanol as polar solvents.

A simple, rapid, cost effective and reproducible green chemistry method synthesised alloyed bimetallic (Au-Ag) nanoparticles using O. basilicum leaf and flower aqueous extracts and prepared silver nanoparticles (AgNps) using O. basilicum and O. sanctum leaf aqueous extracts singly and in combination (O. sanctum + O. basilicum). The size, shape and elemental analysis of the nanoparticles was carried out using UV-Visible spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDX), dynamic light scattering (DLS) and zeta potential. Fourier transform infrared spectroscopy (FT-IR) supported by gas chromatography mass spectroscopy (GC-MS) identified the bio-capping agents.

Antidiabetic carbohydrate metabolising enzymes, α-amylase (porcine) and Bacillus stearothermophilus α-glucosidase as models tested the in vitro inhibitory potential of the aqueous and ethanol plant extracts and the phyto-synthesised (Au-Ag) bimetallic and AgNps. In addition, the study investigated the antibacterial potential for the aqueous plant preparations and their respective phyto-synthesised bimetallic and AgNps against the bacterial species Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Salmonella species and Pseudomonas aeruginosa compared to gentamycin and vancomycin.

Results Bimetallic nanoparticles (synthesised from leaf and flower aqueous extracts) displayed inhibitory activity that showed uncompetitive inhibition (leaf extract), and non-competitive inhibition (flower extract) of α-amylase and competitive (leaf extract) and uncompetitive inhibition (flower extract) of α-glucosidase. Bimetallic nanoparticles were higher in inhibitory activity than acarbose and the crude O. basilicum ethanol and aqueous leaf and flower extracts. In the antibacterial analysis, bimetallic nanoparticles derived from O. basilicum leaf showed inhibition against Staphylococcus aureus, Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa and were greater in activity compared to the crude aqueous leaf extract from O. basilicum.

The in vitro inhibitory effect of AgNps derived from O. sanctum and AgNps derived from O. basilicum on both enzymes was higher in activity than acarbose and their respective crude extracts. However, in combination (O. sanctum + O. basilicum), the derived AgNps appeared to be a less potent inhibitor of α-amylase and α-glucosidase enzyme and was lower than acarbose. AgNps synthesised from the combination of O. sanctum and O. basilicum showed the highest percentage inhibition against Bacillus stearothermophilus α-glucosidase, and AgNps derived from O. sanctum and AgNps derived from O. basilicum displayed competitive type of inhibition. In the antibacterial analysis, AgNps derived from the various extracts showed zones of inhibition against the Gram negative and Gram positive bacterial test strains. However, AgNps synthesised from the O. sanctum leaf extract showed higher inhibition against Escherichia coli than the positive control gentamycin and higher inhibition against Staphylococcus aureus compared to vancomycin. In addition, AgNps from O. sanctum leaf extract displayed inhibition against Bacillus subtilis, Pseudomonas aeruginosa and Salmonella species, thus representing the highest antibacterial potential.

Conclusion The results demonstrate the possibility of synthesis of stable silver and bimetallic nanoparticles of Ocimum sp. The synthesised silver nanoparticles and first time synthesis of bimetallic (Au-Ag) nanoparticles displayed enhanced antihyperglycaemic properties compared to their respective crude extracts and, therefore, show promising effects in lowering postprandial hyperglycaemia in diabetic patients with dual potential for antibacterial treatment. However, the antidiabetic and antibacterial effect will need to be further affirmed in a clinical context. Medicinal plants with therapeutic value may create a new platform for further research to explore the potential for herbal medicine and nanoscience as effective biomedical and industrial applications, and for improving existing drug delivery systems in diabetic patients. Investigations into the cytotoxicity of these extracts and phytosynthesised nanoparticles is recommended.