The study of nanotechnology is an intriguing field of current research that is primarily concerned with the production, alteration and application of minuscule particle structures. These structures usually vary in size from ~1 to 100 nanometers. Nanotechnology presents a novel method of technological progress that involves controlling materials on an extremely small scale, which is equivalent to one billionth of a meter (1-3). The field of nanotechnology is a captivating area of expertise that encompasses physics, chemistry, engineering and biology. Over the past few years, nanotechnologies have displayed encouraging outcomes in the domain of human health, especially in the cure of cancer (4).

Nanotechnology is dependent on the production and modulation of nanoparticles (NPs); this process involves marked changes in the properties of metals formed as by-products of combustion reactions. The peculiar properties of NPs render them ideally suited for the design of electrochemical sensors and biosensors (5). At this size, atoms and molecules function differently and have various unexpected and fascinating applications. It provides resources for the production of products, including medical applications (6). NPs have been used in numerous fields, including cosmetics, food medicine and genetics, and have led to a number of discoveries, including fluorescent biological markers, DNA structure testing, tissue engineering, tumor destruction, separation and purification of biological molecules, anticancer agents to tumor sites (7,8). NPs also have antimicrobial activity against pathogenic bacteria, such as multidrug-resistant pathogens (4,9,10).

NPs can be synthesized by using various methods, including physical and chemical approaches. The key physical methods employed to synthesize NPs are evaporation-condensation and laser ablation. Physical synthesis methods provide advantages, such as the absence of solvent contamination in the produced thin films and the uniform distribution of NPs. However, physical methods can be costly, and only a small quantity of powder is produced each time. On the other hand, chemical reduction by organic and inorganic reducing agents is the most common approach for synthesizing NPs using chemical methods; while chemical methods can be expensive, they have low yields and use toxic chemicals (11). Due to issues associated with physical and chemical methods, scientists have turned to biological methods of synthase; biological methods (also known as biosynthesis, green synthesis, biogenic synthesis and biofabricate) provide an environmentally benign, low toxic, cost-effective and efficient protocol to synthesize and fabricate NPs (9,10). These methods employ microorganisms, such as bacteria, fungi, viruses, yeast, actinomycetes, or their by-product and plant extract (12,13).

Fungi are an extraordinary group of microorganisms that are capable of producing a vast array of metabolites. This renders them an ideal candidate for the biogenic synthesis of nanoparticles. The reason behind this is that fungi secrete a substantial amount of extracellular proteins that assist in stabilizing the negative charge of NPs (14,15). Fungal NPs can be effectively synthesized using Fusarium species due to their filamentous nature and easy extraction from plants and soil. This genus is well-studied and can grow on a simple medium at moderate temperatures, rendering it a popular choice for nanoparticle synthesis. The medical applications of NPs synthesized through the Fusarium-mediated method are illustrated in (Fig. 1) (16,17).

The present review discusses the synthesis of NPs using Fusarium species and their potential as antimicrobial agents against multidrug-resistant Gram-negative bacteria. In addition, the current methods for NP characterization are summarized and the mechanisms through which these NPs exert their antimicrobial activity are discussed.

NPs synthesized from fungal sources are used as novel antibacterial and antifungal agents (18). Fusarium is a filamentous, well-studied genus that is widely distributed on plants and in soil. It is easy to isolate this bacterium from soil and it grows on simple media at normal temperatures, as it is not a fastidious microorganism (19,20). Of note, one method which can be used to synthesize NPs from the mycelia of Fusarium spp. fungus would involve growth in Erlenmeyer flasks filled with potato dextrose broth. The flasks would be kept at an ideal temperature of 25±2˚C for 72 h. Once the growth is complete, the mycelia would be collected by filtering them through Whatman filter paper to separate them from the medium and other components. The collected mycelia would then be purified and washed several times with distilled water (21,22).

Subsequently, a suspension would be made by blending the mycelia with distilled water and incubating it again at 25±2˚C for 24 h. After the incubation period was complete, the cell filtrate would be separated by filtration and then treated with varying concentrations of metal salts. The mixture would be left to incubate at room temperature until a noticeable color change was observed (21,22). A list of the Fusarium species that are used in the production of NPs are presented in Table I.

There are numerous techniques used to detect the presence of NPs. The main techniques used for the characterization of NPs are presented in Table II. The primary technique for detecting the formation of silver NPs (AgNPs) or gold NPs is visual observation. For instance, when the fungal cell filtrate changes from yellowish to dark brown, it indicates the formation of silver NPs. Similarly, the solution color changes from yellow to dark red when gold NPs are formed (18,23,24). UV-visible absorption spectra of AgNPs are presented in Fig. 2 [only fungal extract and silver nitrate (AgNO₃) were used as controls in this case].

The scanning electron microscope (SEM) is a highly versatile instrument that allows for the examination and analysis of microstructure, morphology and chemical composition. The naked eye can only distinguish objects subtending about 1/60˚ visual angle, corresponds to a resolution of ~0.1 mm (when viewed from an optimal distance of 25 cm). Optical microscopy can enlarge the visual angle through its lens, although it has a resolution limit of ~2,000 Å (25). The NPs are observed by transmission electron microscopy (TEM) characterization and are cleaned through plasma treatment using oxygen for <1 min. The sample is placed on the grid and allowed to dry at room temperature. The samples are then inspected by operating at 120 KV (16). An example of the characterization of NPs using both SEM and TEM is illustrated in Fig. 3.

The UV-visible spectrophotometer is a method used to detect NPs shown in Fig. 2. To analyze the NPs, their liquid samples were scanned in the range of 200-800 nm, with fungal filtrate and AgNO₃ used as controls. A scattering cell was used in this range, through which a laser beam (~40 mW at k=635 nm) was passed. To observe the nanoparticles via the path of the laser beam, a dedicated no-microscope optical instrument (LM-20, NanoSight) was used, which has a charge-coupled device camera. The motion of the particles in the field of view (~100x100 µm) was recorded (at 30 fps), and the subsequent video and images were analyzed to determine the size distribution of the nanoparticles (26,27).

The characterization of NPs can also be carried out using Fourier transform infrared spectroscopy (FTIR). FTIR aims to analyze the biomolecules responsible for reducing silver ions and stabilizing NPs in the solution, as illustrated in Fig. 4. For the sample preparation, a colloidal NP solution would be mixed with potassium bromide (KBr) in a clean crucible until a fine powder is produced. The dried powder of NPs is then prepared and dried in an oven to remove any traces of moisture and analyzed in the ranges of 1,000-2,000 cm^-1 at a resolution of 4 cm^-1 (16).

Due to the discovery that some NPs display intriguing antibacterial properties, there has been an increased interest in the manufacture and research of NPs in recent years (8,28).

Antimicrobial resistance poses a major threat to humanity and one of the most severe health crises of the current era. Certain bacterial strains have become resistant to almost all antibiotics, thus rendering it crucial to identify new antibacterial drugs to fight these microorganisms (29,30). In 2017, the World Health Organization (WHO) released a list of priority antibiotic-resistant illnesses, divided into three categories: Critical, high and medium. The majority of the bacteria on the list are Gram-negative pathogens, which are more resistant than Gram-positive bacteria due to their unique structure. This has resulted in a significant global disease and mortality burden (31-33).

Gram-negative bacteria have developed various mechanisms to resist a wide range of antibiotics, such as tetracycline, aminoglycosides and cotrimoxazole. The development of nano-sized particles with antibacterial properties is highly desirable for the creation of novel pharmaceuticals (28,34). Consequently, scholars are actively exploring alternatives to conventional antibiotics in response to rising antibiotic resistance. Their investigations encompass a diverse range of solutions, including plant extracts known for their antimicrobial properties, the development of new antibiotic derivatives with enhanced efficacy, and the use of NPs that can target bacteria more precisely (35,36). However, there has been insufficient research conducted on the toxicity of NPs, particularly regarding their mechanisms of action. This is a matter of concern, particularly as the field of nanomedicine continues to grow. In recent decades, NPs have been widely used in various industries, including as food additives and for drug delivery purposes (37). With the persistent rise of bacterial resistance, there is a growing need for the development of new antibiotics. One of the most promising emerging antibiotic drugs is metal NPs, which have demonstrated potent antibacterial action in the majority of trials (7,38).

In general, smaller NPs tend to exhibit greater antibacterial activity. However, there are conflicting findings regarding the effectiveness of larger-sized NPs, and size alone is not always the most critical factor in determining their toxicity. Other factors that can affect the antibacterial properties of NPs include the formulation process, the surrounding environment, bacterial defense mechanisms and the physical properties of the NPs themselves (39).

NPs that are smaller in size have a higher surface area-to-volume ratio compared with larger NPs. This explains why they are more toxic than larger ones. A larger surface area of small NPs increases the proportion of contact with bacterial cells. NPs <10 nm in size have a higher proportion of contact with bacteria. The interaction between the NP and bacterial surface causes an electrical impact that enhances the reactivity of NPs (38,40,41). Various-sized silver NPs have different antimicrobial activities against different Gram-negative pathogenic bacteria, as illustrated in Table III.

There are various theories regarding the specific mechanisms of the antibacterial action of AgNPs. However, the precise mechanisms involved continue to be under investigation. Some proposed possibilities include the generation of free metal ion toxicity from the surface of synthesized nano-metals and oxidative stress from the reactive oxygen species (ROS) on the surface of NPs (42-44).

NPs can cause a deletion activity on the cell wall of organisms, reduce oxidative stress, inactivate protein synthesis, penetrate the cell membrane and modify essential proteins, thereby increasing cell signal processes and hindering the formation of biofilm (17,45). The mode of action of NPs in damaging the bacterial membrane, bacterial protein and bacterial DNA remains a topic of research. The possible mechanism relies on the interaction of NPs with bacteria, excessive ROS generation and the precipitation of NPs on the bacterial exterior; which disrupts the cellular activities, resulting in membrane disruption (44,46,47).

NPs can cause intracellular alterations by inhibiting DNA replication ability. NPs which are <10 nm in size can diffuse into the nucleus, causing DNA damage, chromosomal abnormalities and cell cycle arrest, leading to genotoxicity in human cell lines (17,48).

NPs have different mechanisms against Gram-negative pathogens. Nano-silver can interact with the bacterial membrane, which is considered the main mechanism for its antimicrobial toxicity. NPs anchor themselves to the bacterial membrane, penetrate it and trigger the destruction of the cell membrane. This can lead to the release of ions into the intracellular medium, which further increases the toxicity level. NPs located on an Escherichia coli membrane can hold fast to the bacterial cell wall, enter it, and cause structural changes in the cell film such as the penetrability of the cell layer (17,48). Copper NPs and zinc oxide NPs can also cause damage to the bacterial membrane (17,43).

Gold NPs, for example, exploit their antibacterial powers against multidrug-resistant (Gram-negative) bacteria via two ways: Degrading membrane potential to lower ATP levels by reducing ATPase activity and preventing ribosomal subunit interaction with tRNA (43).

Metal reacts with sulfhydryl group proteins in cells, inactivating the proteins. Silver ions and AgNPs can interact with chemical groups like sulfide and chloride (43). NPs have different mechanisms of action against Gram-negative pathogens, as illustrated in Fig. 5).

NPs derived from Fusarium species provide a cost-effective and eco-friendly alternative to traditional physical and chemical synthesis methods, demonstrating prominent antimicrobial properties, particularly against Gram-negative bacteria. They can be easily produced using simple media, rendering them an appealing option for large-scale manufacturing. However, challenges include variations in size and stability, the potential for fungal contamination, and the necessity for comprehensive toxicity assessments to guarantee safe clinical application.

The present mini-view demonstrates the immense promise of biological synthesis for the production of NPs. In particular, the production of NPs using fungi that are produced from Fusarium species has a wide range of applications. These NPs are noteworthy for their potent antibacterial action against a range of pathogenic bacteria, including those that show resistance to several drugs.

Further research on the mechanisms of action of NPs produced from Fusarium against bacteria resistant to several drugs is essential going forward. By conducting thorough research in this field, it is possible to deepen the understanding of the underlying mechanisms and develop more effective strategies to address the increasing problem of antibiotic resistance.