Abstract: Biorecycling microbes, which have critical functionalities in natural cycles, are essential to sustain ecosystem of the earth. Any alterations in these cycles caused by the mutations of microbes could be a potential threat to life on earth. Antibiotics leached from pharmaceutical waste, animal food and agribusiness products are accumulating in the environment. Metal nanoparticles are also accumulating in environment because of their extensive use as biocidal agent in domestic products. Interaction of antibiotics and metal nanoparticles with eco-friendly microorganisms has a potential to alter the ecosystem of the earth. In this article, we have studied the antibacterial activities of silver and copper nanoparticles and their formulations with antibiotics, tetracycline, and kanamycin against biorecycling microbes, Bacillus subtilis and Pseudomonas fluorescens. Strong synergistic effect of metal nanoparticles on the antimicrobial activities of commercial antibiotics has been observed. Antimicrobial activity of tetracycline improves by 286%-346% and 0%-28% when being tested in the presence of 250?ppm of silver and copper nanoparticles, respectively. For kanamycin, the improvement is 154%-289% for silver and 3%-20% for copper nanoparticles. Irrespective of the antibiotics and tested organisms, synergy is more prominent for silver nanoparticles even at their minimum active concentration (100?ppm). This study demonstrates that the combination of metal nanoparticles with antibiotics could be more fatal to ecosystem than either the metal nanoparticles or the antibiotics alone.

Key words: Synergy, Nanoparticles, Antibiotics, Biorecycling microbes, Antibacterial activities