Antibacterial effect of some essential oils administered alone or in combination with Norfloxacin

The objective of the present study was that of verifying a possible synergistic antibacterial effect between Pelargonium graveolens [Lis-Balchin, M., Deans, S.G., Hart, S., 1996. Bioactive Geranium oils from different commercial sources. J. Essential Oil Res. 8, 281–290.] essential oil (and its main components) and Norfloxacin antibiotic.As a first step growth inhibition by some types of essential oils was assessed in five microbial species. The antimicrobial effects of P. graveolens oil, as well as those of its components, were evaluated by means of the agar dilution method (ADM) against Bacillus cereus ATCC 11778, Bacillus subtilis ATCC 6633, Escherichia coli ATCC 35218, Staphylococcus aureus ATCC 6538 and S. aureus ATCC 29213.The results obtained highlighted the occurrence of a pronounced synergism between P. graveolens essential oil and Norfloxacin against three of the five bacterial species under study with a FIC index in the 0.37–0.50 range. Such antibacterial effects were also shown to increase, although to a lesser extent, when Norfloxacin was given with the main components of P. graveolens essential oil.Significance and impact of the studyThe combination of Norfloxacin with either P. graveolens essential oil, or with some of the main components of this latter, in the treatment of infections caused by some bacterial species is likely to reduce the minimum effective dose of Norfloxacin thus minimizing the side effects of the antibiotic.