Since 1928, that Alexander Fleming discovered penicillin as the first true antibiotic, for many years, different antibiotics have been effective medical tools for treating bacterial infections. However, with overuse of antibiotics, the bacterial evolution has been accompanied by the emergence of antibiotic resistant strains, which has led to a worldwide crisis in the battle against bacterial infections. Bacteria may acquire resistance to antibiotics by different mechanisms including drug extrusion through efflux pumps such as NorA and NorB to neutralize the bactericidal effect of antibiotics. Staphylococcus aureus is one of the most common pathogens responsible for nosocomial infections that the most important universal health problem with it is the emergence of multi-drug resistance strains. Accordingly, the discovering, production, and prescribing of new antimicrobial compounds have become one of the most important concerns of researchers. One of the potential applications of nanoparticles that studied in several works is the antimicrobial property. The antibacterial activity of silver nanoparticle as well as Fe3O4 nanoparticles had been previously studied and widely described. Synthesis of nanocomposites from the combination of two types of nanoparticles with antibacterial effect increases the performance of both materials and makes their application wider. Although there are several methods to synthesize nanoparticles including chemical and physical methods, but these methods are generally expensive and may carrying traces of hazardous substances, which limits their use in clinical and human applications. Today, the development of green synthesis methods for nanomaterials is an important aspect of research in the field of nanotechnology. Here, we report the biosynthesis and characterization of Fe3O4/Ag nanocomposites by Spirulina platensis cyanobacteria and their impacts on the expression of efflux pump genes in ciprofloxacin resistant S. aureus. The physical pr
