To isolate the mechanisms involved, the researchers used pristine, lab-grown SWNTs with a well-defined size distribution (diameter 0.75–1.2 nm). Commercial SWNTs are often treated with strong acids and can contain 4.5–15% metal and other impurities.

"Some people have suggested that metal impurities may contribute to the toxicity of nanomaterials, but we witnessed severe damage to the cell wall of E. coli K12 incubated with clean SWNTs for 60 minutes," Menachem Elimelech told nanotechweb.org. "I should add that our unpublished work indicates that commercial SWNTs also exhibit antimicrobial activity comparable to the behaviour shown in the paper, which may support our proposed mechanism of physical cell damage through direct contact."

Scientists are making progress in unravelling the toxic effects of nanomaterials. Earlier this year, nanotechweb.org covered the work of researchers in India who recorded images of antibacterial silver nanoparticles embedded in gram-negative structures such as E. coli (see related stories). Both groups are keen to use their research as a means of reducing infection and preventing the transmission of disease, but the results will no doubt raise concerns over safety.

"Because of the potential health effects, SWNTs should be treated as toxic materials," cautioned Elimelech. "Considering the large aspect ratio (length to diameter) of SWNTs, they may exhibit asbestos-like effects upon inhalation."

Of course, it's not all doom and gloom as there are real benefits up for grabs. Elimelech thinks that SWNTs could be useful building blocks for antimicrobial materials, reducing the need for disinfectant and improving hygiene. The toxicity of SWNTs could be restricted by locking the material within a matrix to prevent it from leaching into the environment.

Back in the lab, the group is busy gathering more data. "We're now studying the toxicity of multi-walled carbon nanotubes and our preliminary results show that they are less toxic than SWNTs," he reveals. "At the same time, we are also looking at the effects of SWNTs on a wide range of bacterial strains to better understand the mechanism of cellular damage."

The researchers presented their work in Langmuir.