Introduction Through past years, the serious problem of public health was and still is bacterial resistance which is increasing mostly because of overtaking antibiotics. bacteria can genetically gain resistance and be resistant to variety of antibiotics and drugs. for a long time, plants have been used to maintain human health and in last decade, the use of herbal medicine has grown and shown a significant result because of their natural and effective components which bacteria are less resistant to them. in this study we investigate the antibacterial effect of geranium oil by well diffusion method. geranium oil is the extract from pelargonium graveolens plant and was bought from guangzhou hengyu flavors & fragrances co. and it contains citonellol, nerol and geraniol. the bacteria that used for this study was e coli, pseudomonas aeruginosa, staphylococcus aureus and bacillus Methods To prepare the bacteria, first all bacteria were incubated in nutrient broth media for 24h and then cultivated in nutrient agar media with wells which have been filled with 25Âμl of geranium oil, then after 24h of incubation the inhibition zone was measured in millimeter by precise ruler. disk diffusion method was used for synergism effect for geranium oil with penicillin and cephalexin disks. for this method, e coli and pseudomonas aeruginosa were cultivated in nutrient agar media and then the disks were added at top of the plates. Results Geranium oil inhibition zone was 15mm for e coli, 25mm for bacillus and 12mm for staphylococcus aureus but geranium oil has no effect on pseudomonas aeruginosa and there was no inhibition zone. penicillin inhibited pseudomonas aeruginosa with 8mm diameter and inhibited e coli with 9mm and cephalexin inhibited e coli with 18mm diameter and had no effect on pseudomonas aeruginosa. the synergism result showed that if geranium was added to penicillin disk in e coli the inhibition zone shown 3mm increase and for pseudomonas aeruginosa shown 12mm increase an
