Bacterial biofilms, and especially those made up of multi-drug-resistant bacteria, have become a severe threat to public health in recent years and are very difficult to eradicate. The best known of these “superbugs” is MRSA, or methicillin-resistant Staphylococcus aureus. These robust biofilms frequently occur on synthetic implants, such as urinary catheters, artho-prostheses and dental implants. They can also proliferate on living tissue.

Phytochemicals could be promising and potent alternatives to traditional antimicrobials to treat antibiotic-resistant bacteria, says team-leader Vincent Rotello, and these essential oils and natural compounds are also particularly interesting as “green” antimicrobials, thanks to their low cost and the fact that they are biocompatible. However, their main drawback is that they do not generally dissolve very well in aqueous solutions, which makes administering them difficult, and they cannot be stored for too long .

Researchers have already shown that essential oils can be made more soluble in water by encapsulating them in colloidal delivery vehicles that have been stabilised by surfactants. The problem is that these carriers often irritate the biological tissue they are meant to treat.

An emulsion of essential oils encapsulated in silica nanoparticles

Now, Rotello and colleagues say they may have found a way to overcome all of these challenges: by creating an emulsion of essential oils (in their case peppermint oil and cinnamaldehyde) and then encapsulating it in silica nanoparticles. “All of the steps required to make the capsules are easy to do in the lab and could even be readily scaled up to make industrial quantities,” Rotello says, “To treat a biofilm, we simply apply the capsules to it and they penetrate it and kill it.”

Antibacterial, but not only

The Amherst team found that the capsules effectively eradicated both a laboratory bacteria strain (E. coli DH5R) and three clinically isolated pathogenic bacteria strains – namely Pseudomonas aeruginosa (CD-1006), Staphylococcus aureus (CD-489) and Enterobacter cloacae (En. cloacae) complex (CD-1412). The capsules were also able to treat both Gram negative (E. coli, P. aeruginosa, and En. cloacae complex) and Gram positive (S. aureus) bacteria. The researchers followed the disinfecting process using confocal microscopy, X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).

And that was not all: the capsules also appeared to promote signalling of insulin-like growth factor-I in the mammalian cells being treated. This growth factor increases the number of fibroblasts, which play a vital role in the wound-healing process, helping to close the injury and redevelop the extracellular matrix within skin, explains Rotello.

New weapon in the battle against MRSA?

He reckons that the first and most direct application for the capsules will be as disinfectants for wounds and implants, however, thanks to their excellent antimicrobial activity. “We are even more excited that they might be used as a new weapon in the battle against MRSA in the longer term,” he told nanotechweb.org.

The researchers are now busy testing their capsules in an animal model. “These experiments will allow us to see how the system works on a real-world biofilm and find out if the enhanced fibroblast proliferation we observed in the lab indeed translates into accelerated wound healing,” adds Rotello.

The research is described in ACS Nano DOI: 10.1021/acsnano.5b01696.