Antibacterial activity of the essential oils of catnip (Nepeta cataria L.) and lemon balm (Melissa officinalis L.) against clinical isolates from the respiratory tract

Abstract

Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the essential oils of catnip (Nepeta cataria), lemon catnip (N. cataria var. citriodora) and lemon balm (Melissa officinalis), whose composition has been analyzed by GC-MS, were determined in vitro by a modified broth microdilution method according to the German DIN-regulation 58940–8 [1] against clinical isolates of Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis. These bacteria are frequently involved in respiratory tract infections like sinusitis, tonsillitis, otitis media, bronchitis and pneumonia. Of each strain twelve patient isolates from different culture materials were chosen, which had been identified and characterized by an antibiogram in the routine laboratory of the Hygiene Institute, University Hospital Heidelberg. In spite of their different origin and level of resistance to standard antibiotics, all isolates were susceptible to catnip and lemon balm oils with MIC values ranging from 0.25 % to 0.008 % (v/v). MIC values within the groups of isolates did not differ from those obtained with essential oil sensitive reference strains by more than one dilution step, indicating that natural resistance to these essential oils and cross resistance to standard antibiotics are unlikely to occur in these bacteria. Lemon balm oil, whose main components were geranial (23 %) and neral (17 %), exhibited the highest antibacterial activity, followed by lemon catnip oil, whose main components were the monoterpene alcohols nerol, citronellol, and geraniol (≈ 50 %), and catnip oil which contained mainly nepetalactones (77 %) and only small amounts of monoterpene aldehydes.