Objective
Pittosporum angustifolium Lodd. is used to treat a variety of pathogenic diseases and inflammation by Australian aborigines. Practitioners of complementary medicine frequently use herbal medicines concurrently with conventional antibiotics. There is a need to evaluate their effects in combination.

Methods
The bacterial growth inhibitory activity of P. angustifolium leaf extracts was assessed against a panel of pathogenic triggers of some autoimmune diseases by standard disc diffusion and liquid dilution minimum inhibitory concentration (MIC) methods. Combinational effects between the extracts and conventional antimicrobials were classified using the sum of the fractional inhibitory concentration. Synergistic interactions were further assessed across a range of ratios by isobologram analysis. The toxicity of the individual samples and combinations was evaluated by Artemia lethality and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) human dermal fibroblast cell viability assays.

Results
P. angustifolium leaf extracts strongly inhibited the growth of several bacterial triggers of autoimmune diseases. The methanolic, aqueous and ethyl acetate extracts were particularly good inhibitors of Proteus mirabilis and Klebsiella pneumonia growth (MIC?=?26 and 57?μg/mL respectively). Some combinations of the extracts and conventional antibiotics significantly potentiated the combined inhibitory activity compared to the individual components. Of the 250 combinations studied, approximately 0.02% showed synergistic interactions, 49.6% were additive, 46.8% showed indifferent interactions and antagonism occurred in only 0.02% of the combinations. Interestingly, all of the synergistic and antagonistic combinations contained tetracycline as their antibiotic component.

Conclusion
P. angustifolium leaf extracts inhibit the growth of pathogenic triggers of some autoimmune diseases. Some extracts also potentiated the activity of conventional antibiotics, without significantly affecting the toxicity of the combination.

Key words: Medicinal plants, Conventional antimicrobials, Synergy, Interaction, Toxicity