Infection of implants by microbial biofilm is chiefly caused by Staphylococci, Pseudomonas and Candida species. The growth of microbes by forming biofilms offers them protection from antibiotics, drugs and host defense mechanisms. The eradication of biofilms from implants and medical devices is difficult because of the protection by the biofilm forming pathogenic microbes. Hence, researches are focused on development of antibiofilm materials, which are basically constituted of antimicrobial substances or antimicrobial coatings. Nanomaterial-based coatings offer a promising solution in this regard. Quantum dots (QDs) are the group of semiconductor nanoparticles with high photoluminescent properties compared to conventional organic fluorophores. Thus, drug-conjugated QDs can be a promising alternative for biofilm treatment, and these can serve as excellent alternatives for the mitigation of recalcitrant biomaterial-associated infections caused by resistant strains. Furthermore, their use as antibiofilm coating would avoid the dispersion of antimicrobial agents in the surrounding cells and tissues, thereby minimizing the risks of developing microbial resistivity.
- quantum dots
- microbial biofilms
- antibiofilm materials
Quantum dots (QDs) represent a class of colloidal semiconductor nanocrystals having fluorescent properties that absorb photons at a particular (lower) wavelength and emit at a higher wavelength. These QDs are basically composed of a core and corona layer. The photoluminescence emission wavelength of QDs is directly proportional to its size. The core of the QDs may contain one or more heavy elements such as cadmium, selenium, zinc or tellurium. QDs possess significant superiority over the conventional fluorophores in terms of physicochemical and fluorescent properties. The distinguishable fluorescent properties, smaller size, photostability, resistivity to metabolic degradation and capability of conjugation to ligands/biomolecules make QDs a superior choice for biological applications compared to conventional fluorophores.
In the last three decades, several microbes (fungi, yeast and bacteria) have emerged as major human pathogens and have been responsible for causing life threatening diseases especially in immunecompromised individuals and patients with serious medical issues . The widespread and prolonged use of antifungal agents and drugs for treating the infection caused by the pathogens has resulted in increasing incidences of multidrug resistance (MDR). Additionally, several mutant strains have developed that show high resistance to the antifungal drugs being used . For example,
Microbial communities adhere to a solid surface especially in surface/water interference forming biofilms . Microbes attach to the surface by means of extracellular polymeric substances (EPS), and this acts as their survival means against harsh environmental conditions. Biofilm formation is however associated with surface deterioration and corrosion. In addition, pathogenic microbes form biofilms on medical devices and implants, and this has become a great concern in the arena of healthcare. Biofilm also enhances microbial activity and provides protection against harsh environmental conditions such as drugs, antibiotics and common sanitizers. Because of the emerging conditions of MDR, there is a demand for developing new drugs, antimicrobial agents and modifiers capable of inhibiting microbial growth and biofilm formation. With the necessity of developing antimicrobial agents with diverse functionality and ability to kill both strains of bacteria, nanomaterials have been widely investigated in this regard. Silver nanoparticles , copper oxide nanoparticles [11, 12, 13], metal oxide nanoparticles [12, 13] and even carbon nanomaterials  have been reported for their excellent antimicrobial efficiency. Among these, silver nanoparticles have been extensively used as antimicrobial and antibiofilm agents due to their broad spectrum antimicrobial activity, multiple cellular targets and minimum host toxicity. However, high concentration of silver is toxic to humans and its persistent use causes argyrosis and argia [15, 16]. Hence, the demand is for exploring novel nanomaterials with effective antimicrobial and antibiofilm properties along with biocompatibility. Therefore, the requirement must be targeted towards exploring novel biocompatible nanomaterials with effective antibiofilm and optical properties. QDs can be suitable alternatives because of their intriguing optical, fluorescence, high quantum yield, photostability and easy conjugation efficiency. QDs easily attach to microbial surface because of their small size and their dispersion stability is basically governed by colloidal theory . These are excellent candidates in biomedical applications such as imaging, diagnosis and sensing and drug discovery. Developing QDs-based nanocomposites as coating materials on implants and catheters can thus combat pathogenic invasion and biofilm formation. QDs could be engineered with coating agents and conjugated with bioactive ligands or biorecognition elements for targeted treatment, biofilm visualization, and inhibition.
2. Biofilm formation, its mechanism and transmission
Biofilm can be defined as microbial cells enclosed in an exopolysaccharide matrix and adhered to a cell surface. Formation of biofilms by bacteria and fungus is a defense strategy for protection from environment. Microbes secrete extracellular polymeric substances (EPS) that act as a primary scaffold for attachment to solid substrate  and its basic constituents are proteins, polysaccharides, nucleic acids with some lipids and humic substances . Three-dimensional study of the EPS layer suggested that it forms a gel-like network wherein microbes are embedded and it also maintains the attachment of bacteria to the solid substrate . Stability to the 3D structure of EPS is rendered by the hydrophobic interactions as well as van der Waals attraction between amino acids/peptides and cations such as Ca2+ and Mg2+ . Biofilm formation and its structure depend on the environmental conditions to which the bacteria are exposed. When cells are in a nutrient stress condition, an increase in EPS secretion occurs, which promotes hydrophobic interactions to allow attachment to solid substrate . It has been suggested that the presence of a high concentration of EPS negatively affects the diffusion of lipophilic compounds (such as sanitizers, antibiotics and hydrocarbons), across the microbial cell surface [23, 24].
3. Role of QDs in inhibiting biofilm formation
Biomedical implants are a necessity in modern health care; biofilm formation on these implants and devices is a major cause of their failure. Mostly
Aqueous solubility and compatibility make graphene quantum dots (GQDs) useful in biomedicine. GQDs are reported to be biocompatible at cellular levels investigated via WST-1 assay, LDH production, ROS generation and
Furthermore, the use of semiconductor QDs will allow visualization of biofilm inhibition due to their fluorescent properties. The current methods being used for biofilm analysis are SEM, AFM, MRI and Raman spectroscopy that require lengthy and costly procedures apart from sample modulation, which sometimes provide partial details of the samples concerned [43, 44]. Other than this, conventional fluorescent dyes conjugated with carbohydrate recognition elements are used for biofilm analysis via confocal laser microscopy . However, the use of a synthetic complex is sometimes toxic to cells thereby preventing in situ analysis. Therefore, QDs can be an exceptional solution for this. Moreover, amphiphilic carbon dots (CDs) have been shown to penetrate the EPS layer of
Figure 2 presents the mode of action of quantum dots. The application of QDs as antibiofilm agents can inhibit microbial biofilm at two stages. It can act at the initial stage, where its presence would hinder further attachment of microbial cells to the solid substrate thereby preventing the progression to mature biofilm stage and EPS secretion. Secondly, QDs can act on the matured biofilm, where its penetration into the cells would result in killing of the microbes and subsequent dispersion of the formed biofilm.
With this, we envision that QD-based antibiofilm coatings can be promising probes in investigating biofilm imaging, treatment and their eradication. Furthermore, their broad spectrum activity and minimal host toxicity are additional advantages in this regard. Hence, the use of semiconductor QDs would not only allow detecting the inhibition process but also favor their visible monitoring.
4. Conclusion and future perspective
There is a steady increase in the use of QDs. Despite the several advantages offered by QDs, with some improvements, these can emerge as excellent probes for biological applications. Focus should be towards improved protocols for functionalizing the surface of QDs simultaneously making sure that its properties remain unaltered and secondly, appropriately modifying the surface of QDs so that they do not aggregate in a protein-rich solution or cystol. These methods along with the said advantages would assist in utilizing QDs for biological and biomedical applications. Furthermore, the QDs can be tagged with antimicrobial drugs or drugs can be encapsulated inside the QD core thereby increasing the potency of drugs even at low concentration. Synergistic effect of silver nanoparticles with antibiotics such as penicillin G, amoxicillin, erythromycin, clindamycin and vancomycin is known. Therefore, studies on the synergism between QDs and drug molecules have to be analyzed in detail. This would also assist in providing insights into the molecular mechanism of action of QDs and any kind of cellular changes occurring in the pathogen upon its interaction with pathogenic microbes. Additionally, QDs labeling would allow a high throughput analysis of biofilm inhibition and disruptions that will have significant effect in healthcare sector to identify and combat biofilm formation and pathogenic infections.
This research was supported by DST-PURSE-II funding. KR acknowledges the receipt of a DST-Inspire Faculty award from Department of Science and Technology, Government of India.