While this isn't the first time that graphene-oxide's antimicrobial properties have been reported, the researchers, who were based at the Catholic University of the Sacred Heart and the Institute for Complex Systems in Rome, say that previous results have been contradictory. Instead, they aimed to analyse how the size and concentration of graphene-oxide sheets affects its antimicrobial action. Lead researcher Valentina Palmieri explains that graphene-oxide's antimicrobial effectiveness "depends on its size, bacterial exposure to the material and other experimental considerations".

The team examined the effect of graphene oxide on three bacteria: Staphylococcus aureus and Enterococcus faecalis, which both cause various opportunistic and hospital-acquired infections, and Escherichia coli, which can cause serious food poisoning. They found that 200 nm sheets of graphene oxide in a water solution killed around 90% of S. aureus and E. faecalis, and around 50% of E. coli in less than two hours. Graphene oxide was effective against bacteria, even at concentrations below 10 μg/ml.

There are three components to the material's antibacterial properties. "Graphene-oxide sheets can cut bacterial membranes acting as a nano-knife, wrap the bacteria as a blanket stopping their growth, or oxidise bacterial cellular components," says Palmieri. The team also found that graphene oxide was effective against the fungus Candida albicans, which causes opportunistic fungal infections, such as thrush, with a similar efficacy as found with E. coli.

Tough on resistance

Antibiotic resistance is an increasing problem. Around 25,000 people in Europe die every year from resistant bacterial infections. Without new antibacterial agents, routine medical procedures and operations could soon become impossible. Palmieri says that while bacteria rapidly evolve resistance to antibiotics, the "antibacterial mechanism of graphene oxide, based on both mechanical injury and chemical oxidation" makes it hard for resistance to develop.

Graphene oxide is easy and cheap to manufacture by oxidising graphite. The researchers also point out that graphene oxide can be mixed with biocompatible polymers to make an antibacterial coating suitable for medical equipment susceptible to bacterial colonisation, such as catheters. Surgical tools coated with the material, for example, could kill bacteria. This could reduce the need for antibiotics, decrease post-operative infections and cut recovery times. The team's research was part of an EU-funded project, dubbed VANGUARD, which is testing the feasibility of using graphene-oxide scaffolds to help repair and regenerate damaged tissue and organs.

"These applications could be revolutionary for the management of hospital-acquired infections, especially for immunocompromised patients," says Palmieri. "The low cost of graphene oxide makes it particularly relevant for infection control in low-income countries."

Laura Piddock, director of Antibiotic Action and the public engagement chair at the British Society for Antimicrobial Chemotherapy, told physicsworld.com that the research is "interesting and could provide the basis for a novel preventative agent. However, there is a very long way to go to show that this can be turned into a medicine that is both safe and efficacious". Palmieri's team now plans to test the effect of graphene oxide on other pathogens, such as antibiotic-resistant bacteria, like Methicillin-resistant S. aureus (MRSA), viruses, and other fungi.

The research was presented at the 60th annual meeting of the Biophysical Society, in Los Angeles.