Single-walled carbon nanotubes (SWNTs) are toxic to bacteria because the nanostructures disturb bacterial cell membranes causing the cell content to leak out. The nanotubes may also oxidize the cells, which results in cell death. However, scientists are still debating why SWNTs are so harmful to these microbes.

Chad Vecitis, now at Harvard University, and colleagues have shown that metallic tubes are more toxic to bacteria than semiconducting ones. The researchers came to their conclusions by using a two-dye staining method: one of the dyes only stains the DNA of cells with perturbed membranes and the other dye stains the DNA of all cells. They incubated a culture of E. coli bacteria with the SWNTs and then stained it to evaluate how many cells were still alive.

These results were backed up with scanning electron microscopy images of the bacteria in contact with the nanotubes.

Vecitis' team says that the metallic SWNTs kill more bacteria than the semiconducting ones because they interact more with biomolecules. The researchers show that the metallic tubes oxidize glutathione – an important intracellular thiol – to a greater extent than semiconducting tubes do. This oxidative stress is likely responsible for the greater cellular toxicity of the metallic tubes, says Vecitis.

"Our work shows that the electronic structure of carbon nanotubes must be taken into account when assessing cytotoxicity," he told "This was a topic not addressed in previous studies."

Metallic nanotubes could be used to make antimicrobial coatings for hospitals. The coatings would sterilize surfaces, and so help reduce nosocomial infections among patients. The coatings might also be used to make membranes that reduce biofouling in other environments.

On the downside, SWNT pollution could have detrimental effects on aquatic ecosystems because of their toxicity and more work needs to be done on assessing such risks. "We only evaluated a single bacterial species, E. coli, and so we should determine whether the effects we observed are general or just specific to this microbe."

The work was reported in ACS Nano.