Multidrug-resistant (MDR) bacteria represent a major threat to the success of many branches of medical sciences. Some patients are especially vulnerable of acquiring MDR bacterial infections as a consequence of treatments for illnesses such as organ transplant, hemodialysis, and various types of cancer. Each year at least 150,000 people die around the world due to the infection of a particular MDR bacterium. Therefore, there is an immense need for new strategies to design antibacterial agents.
Nanoparticles (NPs) have been used to synthesize or to improve the remedial efficacy of antibacterial agents[4–10]. NPs for this purpose are generally synthesized by using various metals and polymers[11, 12]. ZnO nanoparticles have been used as an antibacterial agent but, published studies also showed that ZnO nanoparticles were toxic to T cells and neuroblastoma cells[13, 14]. Copper nanoparticles have been shown as a potential antimicrobial agent, but it also comes with the cost of toxicity to eukaryotic cells e.g., potential damage to dorsal root ganglion neuron[15, 16]. Similarly, silver nanoparticles have been studied extensively for their potential antibacterial properties, but many concerns arises over their use on mammalian cells due to the essential toxicity of silver e.g., the genotoxicity towards mammalian cell lines such as mouse embryonic stem cells and mouse embryonic fibroblasts[18, 19]. The use of silver nanoparticles for medical applications is potentially limited due to their nonspecific biological toxicity. Compared to all the metal nanoparticles mentioned above, gold nanoparticles are more amenable to surface modification and are also photostable, and nontoxic based on the extensive review on nanotechnology[20, 21].
Thus, where the undesirable properties such as cellular toxicity and instability of these NPs limit their application, the gold nanoparticles (GNPs) have attracted a significant interest because of their convenient surface bioconjugation, remarkable plasmon-resonant optical properties, chemical stability, and non-toxicity[22–25]. Studies have also shown that the GNPs are useful to improve the efficacy, delivery, target specificity, and biodistribution of the drugs which enhance the antibacterial activity against MDR bacteria[11–14]. However, the use of complicated non-bio/non-ecofriendly chemical synthesis processes and dependence on external sources (such as laser pulses) for the synthesis and/or the activation of GNPs limits their environmental/biocompatibility[26–30]. Moreover, the size of the GNPs strongly influences their physical, chemical, and biological properties[31, 32]. Therefore, there is a need for GNPs with different sizes for various biomedical applications including those with antibacterial activity.
This report is focused on the antibacterial activity of the dextrose-encapsulated gold nanoparticles (dGNPs) which were synthesized by employing a ‘completely green’ method as shown in our previously published article. In this context ‘green’ refers to the process in which the dGNPs were synthesized. The green process is completely natural allowing for the reproducible synthesis of differently sized dGNPs without the need for harsh chemicals or expensive equipment such as lasers. The advantages of said ‘green techniques’ are linked with natural, inexpensive, chemical stability, and ecofriendliness. Three different sizes of dGNPs were synthesized having the average diameters of 25, 60, and 120 nm (±5). The resulting dGNPs were nearly spherical, monodispersed, stable, and water soluble. The dGNPs were prepared using dextrose as both reducing and capping agent.
We explored the antibacterial activity of the dGNPs against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus epidermidis) bacteria. Investigation of the bacterial growth kinetics and growth inhibition, in the presence of dGNPs at various concentrations, was performed using a real-time spectrophotometric assay. Antibacterial activity and efficacy were further validated by turbidimetry and spread plate assays. To understand the mechanism of action, we performed fluorescence microscopy and observed ultrathin slices of nanoparticles-treated bacterial cells under transmission electron microscope (TEM).