comparison of the antimicrobial effects of silver nanoparticles alone and in combination with zataria multiflora extract on some gram-positive and gram-negative bacteria

abstract       background and objective: the spread of drug resistance in bacteria have prompted researchers to seek suitable alternative for antimicrobial drugs among various medicinal plants and nanoparticles. the aim of this study was to evaluate the effect of silver nanoparticles alone and in combination with methanol extract of zataria multiflora on five gram-positive and gram-negative bacteria.      methods: different concentrations of the nanoparticles and extract alone or in combination with each other were tested against the bacteria, using well diffusion method. three concentration levels (lowest, average and highest) were prepared form the nanoparticles and the extract for the combination, and finally nine different combinations were prepared.       results: the extract and nanoparticles showed inhibitory effects against all the tested bacteria. the maximum diameter of growth inhibition zone in the presence of the extract and nanoparticles were observed in streptococcus pyogenes (35.6mm) and methicillin-resistant staphylococcus aureus (20.6mm), respectively. the maximum diameter of growth inhibition zone for the combination was measured in s. pyogenes (31mm).       conclusion: the combination of low concentrations of the plant extract and nanoparticles are more effective against bacteria, but the combination of their high concentrations reduce the antibacterial effects in some cases.