Characterisation and evaluation of antibacterial activity of a new type of essential oil from 'Eremophila longifolia'

Here we report on characterisation of a steam distilled essential oil of the traditional Australian medicinal plant 'Eremophila longifolia' by GC-MS. During collection of this species variant populations of 'E. longifolia' exhibiting unusual morphological variations and an unusual essential oil fraction containing a large proportion of (+)-Menthone, a-terpineol and limonene was discovered growing in a region of Western NSW (Mutawintji National Park). The morphological variations and essential oil compositions reported here have not been previously described in any of the botanical or phytochemical literature pertaining to this species (1,2). Antibacterial MIC values of the oil were determined for 'Escherichia coli' and 'Staphylococcus aureus' using an agar dilution method (3). MIC values for these species were also obtained for various blends of 'E. longifolia aeth.' and Lemon myrtle oil ('Backhousia citriodora aeth.'). The antifungal and antibacterial activity of citral rich lemon myrtle oil is well characterised, citral however is known to produce a sensitisation reaction when applied directly to the skin (3). This reaction has been found to be absent when citral or cirtral rich oils are combined with oils containing α-terpineol. Currently there are efforts to find a-terpineol containing oils that may be blended with lemon myrtle oil without significantly reducing activity (3). Growth inhibition equivalent to 100% lemon myrtle oil was observed with 1:4 ratio blend of 'E. longifolia' and lemon myrtle (MIC: 0.075% v/v, for S. aureus and 0.150% v/v, for 'E. coli').