Antibacterial activity of colloidal copper nanoparticles against Gram-negative (Escherichia coli and Proteus vulgaris) bacteria

Antibacterial activities of as-synthesized nanoparticles have gained attention in past few years due to rapid phylogenesis of pathogens developing multi-drug resistance (MDR). Antibacterial activity of copper nanoparticles (CuNPs) on surrogate pathogenic Gram-negative bacteria Escherichia coli (MTCC no. 739) and Proteus vulgaris (MTCC no. 426) was evaluated under culture conditions. Three sets of colloidal CuNPs were synthesized by chemical reduction method with per batch yield of 0·2, 0·3 and 0·4 g. As-synthesized CuNPs possess identical plasmonic properties and have similar hydrodynamic particle sizes (11-14 nm). Antibacterial activities of CuNPs were evaluated by minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests, cytoplasmic leakage and reactive oxygen species (ROS) assays. MIC and MBC tests revealed dose dependence bactericidal action. Growth curves of E. coli show faster growth inhibition along with higher cytoplasmic leakage than that of P. vulgaris. This might be because of increased membrane permeability of E. coli. CuNP-microorganism interaction induces oxidative stress generated by ROS. Leakage of cytoplasmic components, loss of membrane permeability and ROS generation are the primary causes of CuNP-induced bacterial cell death. As-synthesized CuNPs exhibiting promising antibacterial activities and could be a promising candidate for novel antibacterial agents.