In an article recently published in the International Journal of Molecular Sciences, researchers investigated the feasibility of graphene quantum dots (GQDs) and blue light as a treatment for bacterial infections.
Study: Graphene Quantum Dots Combat Bacterial Infections. Image Credit: Tayfun Ruzgar/Shutterstock.com
Potential of GQDs
Novel nanomaterials are increasingly being explored to address antibiotic resistance, with particular interest in graphene quantum dots (GQDs). GQDs are two-dimensional (2D) graphene structures typically ranging from 10-100 nm in size, renowned for their exceptional physical, biological, and chemical properties.
Despite being composed of a monolayer of carbon atoms, most synthesized GQDs also contain hydrogen and oxygen functional groups. These quantum dots offer several advantages over conventional inorganic quantum dots, including superior biocompatibility, solubility, resistance to photobleaching, robust fluorescence, and ease of synthesis.
GQDs have demonstrated significant potential in various applications, such as biosensing and drug delivery. Recent studies indicate that GQDs can target a range of microorganisms, including pathogenic fungi and antibiotic-resistant bacteria, showcasing their promise as antimicrobial agents.
In addition to their antimicrobial properties, GQDs can generate singlet oxygen under light exposure, making them effective candidates for photodynamic therapy (PDT). Electrochemically synthesized GQDs, for instance, produce singlet oxygen and other reactive oxygen species when exposed to blue light, leading to oxidative stress and subsequent death of human glioma cells.
Thus, antimicrobial PDT using GQDs presents a promising alternative for treating multidrug-resistant microbes. However, the specific effects of these quantum dots on bacterial species remain an area requiring further investigation.
The Proposed Approach
In this study, researchers investigated the photodynamic antibacterial properties of three differently functionalized GQDs—carboxylated GQDs, aminated GQDs, and blue luminescent GQDs—against Escherichia coli (E. coli), a Gram-negative bacterium common in the human intestinal microbiota. E. coli is known to cause a range of infections, from mild gastroenteritis to severe conditions such as sepsis and urinary tract infections.
The objective was to evaluate the antimicrobial potential of GQDs against E. coli through their irradiation-induced antibacterial activity. The study involved treating bacterial suspensions with GQDs and then exposing them to blue light. Specifically, E. coli suspensions were incubated with carboxylated GQDs, aminated GQDs, or blue luminescent GQDs at a final concentration of 200 µg/mL in Luria–Bertani (LB) medium at 37 °C.
Following treatment, bacterial solutions were subjected to blue light irradiation for 15 minutes, 30 minutes, or 1 hour without mechanical agitation and then incubated for 4 hours or 24 hours at 200 rpm under standard atmospheric conditions. Antibacterial activity was assessed by measuring metabolic activity, colony-forming units (CFUs), and the stimulation of reactive oxygen species (ROS).
CFUs were determined at various time points—0, 15 minutes, 30 minutes, 1 hour, 4 hours, and 24 hours—by plating serial dilutions of the bacterial suspensions on LB agar plates. The number of CFUs/mL was calculated by multiplying the number of CFUs on the agar plates by the appropriate dilution factor.
Additionally, scanning electron microscopy (SEM) was used to observe the macroscopic effects of GQDs on E. coli, while the metabolic impact of GQDs was evaluated using the MTS assay in a controlled experimental setup. The cytotoxicity of GQDs was also tested on human colorectal adenocarcinoma cells before establishing the in vitro infection model.
Study Findings
The study successfully demonstrated the antibacterial properties of functionalized GQDs against E. coli, particularly when activated by blue light. The efficacy of GQDs was found to be dependent on the duration of blue light exposure. For example, there was a reduction of up to 2 logs and 3 logs in the CFUs count after 4 hours and 24 hours of blue light exposure, respectively.
SEM analysis revealed that GQDs did not cause significant morphological damage to bacterial cells, suggesting that their antibacterial effect is primarily metabolic. Each type of GQD induced the generation of ROS, which led to oxidative damage to critical bacterial biomolecules, such as lipids, proteins, and deoxyribonucleic acid (DNA). This oxidative stress impaired bacterial metabolism without notably altering cell morphology.
The production of ROS was significantly enhanced by blue light stimulation, which directly compromised E. coli growth. Blue light was effective in PDT because of its ability to penetrate tissues, making it suitable for both deeper and superficial infections. Additionally, blue light's lower toxicity to human cells compared to ultraviolet light underscores its potential for clinical applications. Human colorectal adenocarcinoma cells did not exhibit cytotoxicity when exposed to blue light and/or GQDs.
Furthermore, GQDs were effective in reducing the E. coli burden in infected human colorectal adenocarcinoma cells. They acted in the extracellular environment and disrupted the eukaryotic cell membrane, which facilitated enhanced antibiotic internalization.
In conclusion, the study demonstrated that GQDs, when combined with blue light stimulation, exhibit effective antibacterial activity against E. coli due to their photodynamic properties.
Journal Reference
Rosato, R. et al. (2024). Exploration of the Graphene Quantum Dots-Blue Light Combination: A Promising Treatment against Bacterial Infection. International Journal of Molecular Sciences, 25(15), 8033. DOI: 10.3390/ijms25158033, https://www.mdpi.com/1422-0067/25/15/8033
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