New research has shown that a nanomaterial composed of carbon ‘dots’ could be effectively used in the fight against MRSA.
The work, which was carried out by scientists at Shiraz University of Medical Sciences and Islamic Azad University, revealed that carbon dots, or c-dots, are capable of killing both wild-type Staphylococcus aureus and their methicillin-resistant form (MRSA) – and that irradiating the c-dots was able to enhance this bactericidal effect. The paper will be published in January 2017 in the Journal of Photochemistry and Photobiology.
The c-dots were employed based on a technique known as Photothermal Therapy, used notably to treat cancer. It relies on directing a wavelength towards a nanomaterial, which is able to absorb the wavelength and starts to vibrate. This generates heat, which can be used to kill targeted cells.
For their experiments, the research team grew both wild-type S. aureus and MRSA in vials with different concentrations of c-dots added, and then used an 808-nm diode laser to shine near infra-red light at the c-dots.
They first found that the c-dots on their own were able to decrease the survival of both wild-type S. aureus and MRSA, but that when the near infra-red laser was applied, this killing effect was amplified even further.
They then observed that all concentrations of c-dots were able to inhibit the growth of the bacteria, but when the concentration of c-dots was increased in the vials from 1.7 μg ml-1 to 3.4 μg ml-1, the antibacterial activity of the nanomaterial was increased. At 7 μg ml-1, the bacterial growth was completely inhibited.
The c-dots were then shown to cause protein leakage in the bacteria, and cause them to release Reactive Oxygen Species (ROS). Protein leakage suggests that the structure of the bacterial cells is damaged somehow, which can lead to cell death. ROS are released when cells undergo some form of stress, like heat, and these are linked with damage to cellular structures. The researchers found that the c-dots were able to cause protein leakage at any concentration, but the effect became more extreme as the concentration of the nanomaterial increased. Irradiating the c-dots with near infra-red light seemed to increase this effect. For whatever reason, however, it appeared that protein leakage was greater in the wild-type S. aureus than it was in the MRSA.
ROS release seemed to be triggered by the presence of the c-dots, and again the irradiation of the nanomaterial appeared to enhance this event. The paper noted that since ROS can cause a change in the permeability of the cell membrane, meaning that the cell could allow more molecules to pass from the outside to the inside, it could be possible that the c-dots gain entry to the bacteria in this way, allowing them to cause even more damage.
Another interesting action that the c-dots had is that they seemed to affect the shape of the bacterial cells. Normally, S. aureus cells are smooth and round, forming as clusters with their neighbouring cells. However, when the c-dots were employed, this all changed – wild-type S. aureus appeared to shrink, with damage to the cellular structure and cell wall becoming apparent. MRSA cells became flattened, their cell surface appeared damaged and their typical cluster formation looked as if it had been disrupted.
So what could this research mean for the future? It’s obviously still early days, but c-dots are associated with numerous benefits such as a low toxicity, a good chemical stability, solubility in water and being eco-friendly. This work makes it look as though the natural anti-Staphylococcal abilities of the nanomaterial are boosted by the near infra-red radiation – using them both to target MRSA infections would be beneficial as they don’t rely on targeting specific chemical processes within the bacteria, therefore decreasing the likelihood that resistance would emerge. Future work could potentially see whether this effect could be carried over in in vivo infection experiments.