Rajavel Krishnamoorthy, Gomathi Rajkumar, Manian Sellamuthu, Rajendra Kumar Ramasamy Thangavelu
Advanced Materials and Devices Laboratory, Department of Physics, School of Physical Sciences and ‡Microbiology Laboratory, Department of Botany, School of Life Sciences, Bharathiar University , Coimbatore 641046, India.
Langmuir. 2014 Jan 21;30(2):592-601. doi: 10.1021/la403332b. Epub 2013 Dec 31.
Understanding the bacterial cytotoxicity of CNTs is important for a wide variety of applications in the biomedical, environmental, and health sectors. A majority of the earlier reports attributed the bactericidal cytotoxicity of CNTs to bacterial cell membrane damage by direct physical puncturing. Our results reveal that bacterial cell death via bacterial cell membrane damage is induced by reactive oxygen species (ROS) produced from CNTs and is not due to direct physical puncturing by CNTs. To understand the actual mechanism of bacterial killing, we elucidated the bacterial cytotoxicity of SWCNTs and MWCNTs against Gram-negative human pathogenic bacterial species Escherichia coli, Shigella sonnei, Klebsiella pneumoniae, and Pseudomonas aeruginosa and its amelioration upon functionalizing the CNTs with antioxidant tannic acid (TA). Interestingly, the bacterial cells treated with CNTs exhibited severe cell damage under laboratory (ambient) and sunlight irradiation conditions. However, CNTs showed no cytotoxicity to the bacterial cells when incubated in the dark. The quantitative assessments carried out by us made it explicit that CNTs are effective generators of ROS such as (1)O2, O2(•-), and (•)OH in an aqueous medium under both ambient and sunlight-irradiated conditions. Both naked and TA-functionalized CNTs showed negligible ROS production in the dark. Furthermore, strong correlations were obtained between ROS produced by CNTs and the bacterial cell mortality (with the correlation coefficient varying between 0.7618 and 0.9891) for all four tested pathogens. The absence of bactericidal cytotoxicity in both naked and functionalized CNTs in the dark reveals that the presence of ROS is the major factor responsible for the bactericidal action compared to direct physical puncturing. This understanding of the bactericidal activity of the irradiated CNTs, mediated through the generation of ROS, could be interesting for novel applications such as regulated ROS delivery in cancer therapy and the sanitation of potable water supplies.
了解碳纳米管的细菌细胞毒性对于生物医学、环境和健康领域的广泛应用至关重要。大多数早期报告将碳纳米管的杀菌细胞毒性归因于直接物理穿刺对细菌细胞膜的损伤。我们的研究结果表明，碳纳米管产生的活性氧（ROS）会导致细菌细胞膜损伤，进而引发细菌细胞死亡，而非碳纳米管的直接物理穿刺所致。为了解细菌杀伤的实际机制，我们阐明了单壁碳纳米管（SWCNTs）和多壁碳纳米管（MWCNTs）对革兰氏阴性人类病原菌大肠杆菌、宋内志贺氏菌、肺炎克雷伯菌和铜绿假单胞菌的细菌细胞毒性，以及用抗氧化剂单宁酸（TA）对碳纳米管进行功能化处理后其细胞毒性的改善情况。有趣的是，在实验室（环境）和阳光照射条件下，用碳纳米管处理的细菌细胞表现出严重的细胞损伤。然而，在黑暗中孵育时，碳纳米管对细菌细胞没有细胞毒性。我们进行的定量评估明确表明，在环境和阳光照射条件下，碳纳米管在水性介质中都是诸如单线态氧（¹O₂）、超氧阴离子（O₂•⁻）和羟基自由基（•OH）等活性氧的有效产生者。未修饰的碳纳米管和经TA功能化的碳纳米管在黑暗中均显示出可忽略不计的活性氧产生。此外，对于所有四种测试病原体，碳纳米管产生的活性氧与细菌细胞死亡率之间都存在很强的相关性（相关系数在0.7618至0.9891之间）。未修饰的碳纳米管和功能化碳纳米管在黑暗中均无杀菌细胞毒性，这表明与直接物理穿刺相比，活性氧的存在是杀菌作用的主要因素。通过活性氧的产生介导的对辐照碳纳米管杀菌活性的这种理解，对于诸如癌症治疗中受调控的活性氧递送和饮用水供应卫生等新应用可能具有重要意义。
