Nanotechnology potential in antimicrobial therapy is increasingly demonstrated by various data. Results reveal antibacterial properties, comparable to that of conventional antibiotics. Working on parallel experiments, researchers continue to bring evidence demonstrating age-old-recognized antibacterial properties of various natural components of plant and animal origin. Later years brought an increasing trend for combining synthetic and natural composition in new constructs. The tendency aims to bring more on different essential aspects, such as active substance release, improvement of antibacterial effect, and up-regulation of the mechanisms at the structure-cell interface. Present chapter structures the up-to-date achievements in the field, including the concept of design, biological effects, benefits, mechanisms, and limitations of the field. Also, expected future research directions are to be discussed.
Microbial infections represent a major health problem, being responsible for more than 16 million cases of pathology-related death cases per year . The impact is increased by the appearance of multidrug-resistant bacteria, a mounting tendency, responsible for both acute and chronic forms of clinical presentations of such infections. The need for urgent generation of new, valid therapy solutions, capable of eluding the resistance mechanisms, has been increasingly high during late years. Nanotechnology potential in antimicrobial therapy is increasingly demonstrated by various data. Nanoparticles such as zinc oxide, silver, aluminum oxide, iron oxide, copper, titanium dioxide, and silicon dioxide have been successfully tested by various research groups . Results reveal antibacterial properties, as demonstrated by testing strains of
2. Green synthesis of nanoparticles exhibiting antimicrobial role
The idea of nanoparticle synthesis using green technology represents one of the first, beginning trends in joining the two different domains: nanotechnology and natural extract chemistry. Although the final composition of the nanoparticles designed this way does not necessarily include high concentrations of natural extracts, the concept of green design aims to diminish the risk of possible chemical traces resulting from nanoparticle synthesis in the final product. Reducing or stabilizing agents could be good examples of such traces. In the new green synthesis concept, any traces, if present, would be a part of a natural compound with rather beneficial than dangerous effects. One such report is the synthesis of silver nanoparticles using
However, not only silver nanoparticles have been reported to be successfully synthetized using green technologies. One report provided evidence of triangular gold nanoparticles synthesis by using extract of
A special class of green synthesis designs involves the use of alternative sources for reduction and stabilizing agents, apart from plant or animal origin. Using bacterial, actinomycete, yeast, or fungal strains for providing the necessary material for green synthesis is more recently an interesting technological solution. The use of
3. Nanoencapsulation and microencapsulation of natural compounds designed for antibacterial applications
3.1. Cyclodextrin encapsulation
Increasing the efficiency of natural compounds as well as diminishing their drawbacks, such as limited bioavailability or excessive rate of release, has been one major and constant research topics during late years. Several practical approaches have been designed. Polymer-based nanoparticles as well as naturally derived nanocarriers were the most common experimental trends . Phenolic compounds as well as the specific component piperine are known to be present in black pepper oleoresin. Researchers have started to improve their biological interaction properties by approaching the encapsulation in cyclodextrins. Although the capsules are relying on a natural-based material, their laboratory processing and characterizing the newly designed construct represented an important research step. Testing data also revealed not only higher antioxidant activity for the encapsulated extract, but also more efficient antibacterial effect as compared to nonencapsulated compound. Data revealed that lower concentrations are needed for inhibiting the growth of the Salmonella strain used for evaluation and demonstrated that designed formulation is able to improve the antimicrobial effect of the natural extract . β-Cyclodextrin encapsulation has also been selected by Mourtzinos et al. for optimization of olive leaf natural extract properties. The active component, oleuropein has been already demonstrated to exert anticancer  effects, inhibitory efficiency against certain human pathogens such as Mycoplasma , as well as to provide antioxidant protection , and the obtained formulation offered protection for the natural extract toward better biological effect. Similarly, Dima et al. used an extract coming from
3.2. Complex coacervation
A distinct attempt of providing improved properties for natural extracts by hybrid processing was involving complex coacervation. The extract used was that of propolis, already known as a natural-source food additive. Isolated pectin and soy protein were used as encapsulation material. Although the compounds used in encapsulation were of natural origin, isolation of compounds and the complex coacervation protocol represented a step forward in improving the properties of nanomaterials by encapsulation. The authors have demonstrated the technology to generate a stable, alcohol-free agent in a powder formulation that elicits controlled release properties, but also demonstrated antimicrobial activity against
3.3. Polymer-based encapsulation and liposomes
However, most researchers have focused toward synthetic, polymer-based systems as well as liposomes.
The need for packaging food using materials with antibacterial properties motivated the work of a research team who designed nanocapsules with cinnamaldehyde. The capsules were designed as lipid bilayers of polydiacetylene-N-hydroxysuccinimide (PDA-NHS) nanoliposomes. Immobilization on glass slide was further performed and this type of product demonstrated significant antibacterial activity against
A more extensive study tested various encapsulation designs for active components such as lysozyme, nisin as well as various herbs and spice extracts, including liposomal, chitosan as well as polysaccharide encapsulation. The advantage of liposomal formulation could come from their higher stability compared to chitosan encapsulation. Antimicrobial activity against both positive and negative of Gram bacteria was efficient and stable for a minimum of 1 month. Due to the controlled release possibility derived from the formulation concept, the authors indicate a large potential for applications under hydrogel form with embedded capsules containing natural extracts .
Recently, the synthesis protocols became more oriented toward complex structures, such as polymer-lipid nanoparticles. One of the most robust designs is represented by a core-shell concept, presenting a polymeric core, a lipid shell with embedded active substance, and protected by polyethylene-glycol moieties for immunoreactivity reduction . The advantages of such structures come from increased stability, morphological and structural integrity, low risk of damage during storage, controlled release features, elevated biocompatibility, and bioavailability. Both the polymeric and the lipidic component can be built using not only artificial, but also using natural sources, such as chitosan or natural fatty acids and represent the next generation of materials directed toward antimicrobial applications .
4. Nanoparticles functionalized with natural biomolecules
Not all research groups have followed the encapsulation trend. A part of the research teams have focused on direct attachment of biologically active, natural origin molecules onto the surface of metal nanoparticles. One such design was the synthesis of catechin-Cu nanoparticles. By joining two elements with already known antibacterial effect, the newly formed compound was reported to induce a 3 h-death rate of up to 90 and 85% of
Silver nanoparticles were also reported to have been successfully functionalized with glucosamine, a natural sugar. The newly constructed compound presented high antimicrobial efficiency. Both
5. Mechanisms underlying the antimicrobial effect of natural-synthetic hybrid materials
Although consistent efforts have been made for development of hybrid, natural-synthetic designs, as well as testing their antimicrobial effects, there is still limited data regarding the exact mechanisms involved in the obtained antimicrobial effects. However, the natural compound in the construct can be considered as an important contributor in the final bacterial inhibition mechanism. The most important antimicrobial mechanisms involved in natural extract action, along with studies detailing the effect, are summarized below.
5.1. Membrane permeabilization, membrane potential alterations, and cellular component leakage
One of the most incriminated antimicrobial mechanisms used by natural extracts involves the functional and structural integrity of the membrane. Alteration of bacterial membrane potential demonstrated by Saritha et al. is a study focused on different extracts.
Detailed evidences of protein leakage were brought by a distinct research group, while testing the effects of
5.2. Alterations in regulation of gene expression
The release of bacterial cell content as a result to treatment-induced permeabilization is preceded by enhanced expression of different proteins. Yong et al. have identified several distinct proteins with up-regulated expression following medicinal plant exposure, namely chaperonin (60 kDa), flagellin, triacylglycerol lipase, outer membrane protein A, N-acetylmuramoyl-L-alanine amidase, 30S ribosomal protein s1, and stringent starvation protein A. The paper suggests common antibacterial routes for different natural antimicrobial treatments .
Similarly, evidences provided by El-Hamid et al. support the conclusion of inducing down-regulation of quorum-sensing system. Altering bacterial communication, exerted by plant natural therapies was demonstrated by qRT-PCR and was reported to be induced by down-regulation of quorum-sensing already established genes . Also, transcription processes as well as replication of nucleic acids (DNA/RNA) were reported .
5.3. Metabolic alterations
Besides the already discussed mechanisms, a recent paper has discussed the addition of metabolic-induced alterations by exposure to natural extracts. The mechanisms identified by the authors were respiratory enzymatic inhibition, inducement of oxidative stress, heat-shock state, and forcement of bacterial acute stringent response. The ATP level tends to decrease in the cell, as demonstrated by the
5.4. Effects induced by nanoparticles
The antimicrobial effect of particular nanomaterials represents a complex interaction of distinct effects. Modulation of effects could theoretically come from the cell internalization of free ions resulting from the nanomaterials, cell-nanostructure interaction, and physical properties of the nanostructures such as dimension, morphology, or surface charge. Due to the large area provided by the surface of nanoparticle, the different chemical nature of nanostructures and the final effects are hard to predict and therefore represent a serious research aim for each individual type of nanomaterial . Among antibacterial applications, silver nanoparticles represent a major fraction of tested materials due to widely accepted and traditionally known effects of silver. For this nanomaterial, in particular, effects are mainly due to silver ion uptake, resulting in DNA toxicity and membrane damage .
6. Drawbacks, limitations, and future research trends
The mounting of medicinal resistance in bacteria and the constant changes in bacterial mechanisms against antibiotics trigger the need for different solutions, which would include a natural-based antimicrobial component. Present limitations, however, come from the little interest of pharmaceutical companies in integrating nature-provided elements into their fabrication process. Extraction and testing by specialized companies could provide an additional solution for antibacterial treatment, and should be focused on age-old validated plants used in traditional medicine .
The future of research within the discussed topic is dependent on improved mechanistic understanding at the interface between material and bacterial cell, as well as more in depth knowledge on nanomaterials and their specific behavior in different conditions. The more knowledge acquired, the more complex and tailored the structure of the future constructs will be. Concepts of future structures are becoming themselves a research topic necessary for generation of better nano-antimicrobial constructs .
The advances in nano-antimicrobials based on synthetic-natural (hybrid) designs join the achievements of two domains already demonstrating promising data for future biocide agents. Up-to-date literature suggests acceleration along the path of generating new antibacterial agents, capable to respond to the problem of severe resistance to conventional antibiotics and holding good promises for the future of the domain. Most concepts of synthesis protocols demonstrate practical efficiency, comparable with the standard recommended antibiotic treatment. However, while most polymer-based and liposomal designs were meant for textile and packaging treatment, functionalization of nanoparticles with naturally active compounds seems to suit direct antimicrobial treatment better, including the possibility for adding topics in human-intend applications. Distinctly, capsular products benefit from digestive transit protection of active components, thus making them perfect for oral administration. Concept design results in specific tailoring of final product; therefore, the choice of technology and prototype remains to be made based on the final desired application.
This work was supported by the Romanian National Authority for Scientific Research and Innovation, CNCS-UEFISCDI, project numbers PN-III-P2-2.1-BG-2016-0446, PN-III-P1-1.1-PD-2016-1831, and PN-III-P1-1.1-TE-2016-2161.