Photo of magnetic nanoparticles penetrating the bacteria E. coli. The dark spots are the magnetic nanoparticles.
Photo of magnetic nanoparticles penetrating the bacteria E. coli. The dark spots are the magnetic nanoparticles.

Clusters of bacteria known as biofilms are estimated to be responsible for around 60% of all infectious diseases treated in the West and are becoming increasingly resistant to antibiotic drugs. One problem with biofilms is that they are protected by an outer membrane made up of saccharides, proteins, nucleic acids and lipids, which is difficult to penetrate. Now researchers from Northeastern University and King Abdulaziz University in Saudi Arabia think they may have cracked the problem with magnetic nanoparticles [Geilich et al., Biomaterials 119 (2017) 78].

“We are seeing an increasing number of antibiotic resistant bacteria all over the world and we need to introduce new approaches to kill bacteria,” explains Thomas J. Webster of Northeastern. “When a biofilm forms we are unable to use antibiotics alone because they can not penetrate the biofilm.”

Webster and his team have developed biocompatible nanocarriers that contain superparamagnetic iron oxide nanoparticles (or SPIONs) and a common antibiotic (methicillin). The nanocarriers can be injected near a biofilm and then driven into it using an external magnetic field. Once inside a biofilm, the on-board antibiotic can be much more effective. Meanwhile, the SPIONs can be detected using magnetic resonance imaging (MRI).

“Our new nanoparticle can kill bacteria in a biofilm, can be controlled externally to go wherever an infection is, and can be visualized externally to determine in real time if the infection is decreasing,” says Webster.

The nanocarriers – which can be made from lipid-based materials that resemble a cell membrane or polymer nanoparticles known as polymersomes – can be functionalized to target specific bacteria. The small size of the nanocarriers – and their ability to target bacteria – makes it much easier to slip inside biofilms.

Once inside a biofilm, the nanocarriers and their cargo appear to have a multi-pronged effect. The SPIONs themselves can clog up the biofilm membrane and prevent nutrients from reaching the bacteria inside and waste from escaping. Moreover, iron oxide can increase the generation of oxygen free radicals, which also kill bacteria. Finally, once through the membrane, the antibiotic cargo can reach the bacteria throughout the biofilm.

“This approach is more effective that any existing drug or antibiotic we have tested,” says Webster.

One final advantage of the approach is that while the nanocarriers are deadly to bacteria, they appear to be non-toxic to mammalian cells.

Although the researchers see no major obstacles to the adoption of SPION nanocarriers in the treatment of biofilms, more research and animal studies are needed first, points out Webster. Ultimately, though, the approach could lead to the development of topical skin creams for treating biofilms or coatings for medical devices.