Interaction between the non-volatile and volatile fractions on the antimicrobial activity of Tarchonanthus camphoratus

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The contribution of the volatile constituents to the overall antimicrobial efficacy of the medicinal plant Tarchonanthus camphoratus was considered, where different extraction techniques were applied to yield four fractions. These comprised of the essential oil prepared by hydrodistillation, non-volatile constituents prepared by extraction of plant material remaining in the distilling apparatus (having no or negligible volatile constituents), and extracts prepared from fresh and dried plant material having both volatile and non-volatile constituents. The antimicrobial activities of the non-volatile and volatile fractions of T. camphoratus singularly (MIC method) and in combination (isobologram ratio method) demonstrated that the volatile constituents play an integral role in the total antimicrobial efficacy of the plant. The MIC values for the essential oils of T. camphoratus ranged from 1.5 to 16.0 mg/ml depending on the pathogen studied. With the exception of studies on Klebsiella pneumoniae, the non-volatile fraction devoid of volatile constituents displayed higher antimicrobial efficacies (2.0–4.0 mg/ml). When the volatile and non-volatile fractions were combined, increased efficacy was mostly noted with the dried plant material mostly showing a higher antimicrobially-active profile. Synergistic interactions were further validated by the isobologram studies on the combination of non-volatiles with essential oil.