The aromatogram – the method matters

For specific treatment with essential oils (EO) an aromatogram can be used as diagnostic tool. Hereby, agar disk diffusion is carried out to determine in vitro antibacterial activity of EO.

Briefly, a filter paper disk containing the EO in question is placed on an agar plate previously spread with the pathogenic bacteria. After incubation the diameter of the inhibition zone of bacterial growth is determined. Often pictures of the aromatogram depict testing of several oils on one agar plate, which reflects time and cost-effective routine diagnostic procedure. However, due to synergistic effects between EO, testing of several EO on one agar plate may lead to different results in comparison to single tests.

13 commercially available pure EOs (cinnamon leaf, clary sage, coriander seed, lavender fine, lemon grass, manuka, indian melissa, oregano, palmarosa, ravintsara, tea tree, thyme linalool, thyme thymol) were tested in different aromatogram layouts for their antibacterial activity against clinical isolates of Staphylococcus pseudintermedius, Escherichia coli and Pasteurella multcocida derived from canine or feline samples from routine diagnostic.

Different aromatogram layouts were tested. In the first three layouts several combinations of EO on one agar plate were analyzed, in the fourth layout only one EO per agar plate was tested.

Results show that testing of several EO on one agar plate can lead to larger inhibition zones and thus overestimation of their in vitro activity. This is especially true for gram-positive bacteria like Staphylococcus pseudintermedius and generally sensible bacteria like Pasteurella multocida. Gram- negative bacteria e.g. Escherichia coli seem to be less affected.

In conclusion, analyses of several EO on one agar plate can have an influence on the outcome of the aromatogram, especially testing gram-positive bacteria. A general procedure for laboratories carrying out aromatograms to obtain comparable and reliable results should be established.