Iron-sequestering molecules play a pivotal role on the mechanism of toxicity of SWNTs on fungal and bacterial cells

摘要

For all plants and animals, and for virtually all microbes, life without iron is impossible. Even though iron is the fourth most abundant element in the Earth's crust, it is present in the form of extremely insoluble minerals, which severely restrict the bioavailability of this metal. Microorganisms require iron for a variety of metabolic processes. In response to this, microorganisms secrete high-affinity iron-binding compounds called siderophores. Previous research has revealed that single-walled carbon nanotubes (SWNTs) containing iron particles, interact with bacteria and fungi and exhibit antimicrobial activity. The bactericidal mechanism is still not well understood. Some reports claim that severe cell membrane damage by direct contact with SWNTs is a plausible mechanism. However, in this study we described that iron-sequestering molecules are responsible for the toxicity of SWNTs to microorganisms. After incubating different concentrations of functionalized SWNTs with bacteria and fungal cells, it was found that these microorganisms produced siderophores. These molecules were seen in the medium as early as ten minutes after the addition of SWNTs. After 30 minutes we were able to see some bacterial and fungal cells saturated with siderophore-containing SWNTs. In addition, in the culture medium by itself, we noticed siderophores filled with SWNTs. Further analysis of the bacterial and fungal cells saturated with siderophore-containing SWNTs revealed that those bacterial cells were dead due to the fact that too many SWNTs present in the bacterial cells interfered with processes such as binary fission, and electron transport. Direct contact with the SWNTs is not a possible mechanism in this case since the microorganisms had enough time to produce siderophores and still thrived for a few minutes after the endocytosis of the siderophores. In fact, we noticed that the organisms were able to live for a couple of hours with the SWNTs inside before they started to die. This observation was true for both the bacterial and fungal models used. In addition, we noticed that the effect of siderophore-containing SWNTs in plants had the opposite effect. Instead of dying, the plants were able to grow better. We were able to conclude that high levels of metals such as Al, Cu, and Ni in the soil inhibit Fe acquisition and thus plant growth. Siderophores supply plants with Fe from the SWNTs, which enhanced chlorophyll content and lowered the formation of free radicals. This work provides more insight on the toxicity effects of SWNTs in different organisms and the potential use of SWNTs as a new method to treat bacterial and fungal diseases.