# Anti-Yeast Synergistic Effects and Mode of Action of Australian Native Plant Essential Oils
## 4 Discussion  
The EO of AM showed no antimicrobial activity against the tested microorganisms below 20 mg/mL. [PERSON] and [PERSON] [33] reported that the EO of AM can occur in two  
Figure 6: GC–MS chromatograms of anise myrtle (**A**), Tasmanian pepper leaf (**B**) and lemon myrtle (**C**) essential oils showing their main bioactive compounds. different chemotypes having the same chemical composition but at different concentrations: chemotype 1 contained anethole as the main bioactive compound with a relative concentration of 93 to 95% and methyl chavicol between 4.4 and 5.6%, whereas chemotype 2 contained methyl chavicol as the main bioactive compound (66 to 77%) and anethole as the minor one (20 to 33%). The bioactive compound profile of AM EO in the present study is similar to chemotype 1 reported by [PERSON] and [PERSON], with anethole as the main bioactive compound followed by methyl chavicol at a much lower level. [PERSON] and [PERSON] [61] evaluated the antimicrobial activity of two AM EO samples: the first sample showed no activity against _E. coli_ and _C. albicans_, while the second sample had an inhibitory effect against _E. coli_, _S. aureus_ and _C. albicans_. In addition, a study by [PERSON] et al. [62] reported antimicrobial activity of AM EO against _E. coli_ and _S. aureus_, whereas [PERSON] et al. [63] could not observe any activity against these bacteria. The controversial findings using AM EOs as an antimicrobial agent (effective vs. non-effective) are most likely caused by the reported significant variation in the content and ratio of anethole and methyl chavicol. In contrast to the AM EOs, four different types of LM EOs studied by [PERSON] et al. [64] showed all a strong antibacterial and antifungal activity, which was positively correlated with citral, the main bioactive compound in these four EOs. Unlike the standard antibiotic drugs which possess either an antifungal or antibacterial effect, the tested EOs of LM and TPL had both antibacterial and anti(weak-acid resistant)-yeast activity. Polygodial, the main bioactive compound in TPL EO, was previously reported [65] to possess a stronger antifungal activity than the standard antifungal drug amphotericin B and also showed synergistic effects with antimicrobial drugs [53]. A study by [PERSON] et al. [66] evaluated the inhibitory activity of LM EO and reported MICs of 0.313% against _E. coli_ and 0.156% against _S. aureus_ (complete inhibition after 22 h). In the present study, LM EO completely inhibited the growth of _E. coli_ and _S. aureus_ after 24 h at MBCs of 0.167% (1.67 mg/mL) for _E. coli_ and 0.083% (0.83 mg/mL) for _S. aureus_. Both studies revealed that Gram-positive bacteria were more susceptible to LM EOs than Gram-negative bacteria. Interestingly, the bacterial inhibition concentration of LM EO that prevented the growth of _E. coli_ in the present study was almost the same as that reported by [PERSON] et al. [66] (0.333 vs. 0.313%). Overall, the results of both studies can be used to establish an effective concentration range for LM EO against _E. coli_. Furthermore, the MICs in the present study were similar to the MBCs/MFCs of TPL EO against _D. bruxellensis_, _D. anomala_, _S. pombe_, _R. glutinis_, _S. cerevisiae_, _C. albicans_ and _E. coli_ and for LM EO against _C. albicans_ and _S. aureus_, which clearly indicates that both EOs exhibited a biocidal activity at their MIC levels. The pronounced biocidal activity of TPL EO against bacteria and yeasts, observed in the present study and many other studies, was attributed to polygonali, the main bioactive compound in TPL EO [20, 21, 48, 49, 50, 51, 52, 53, 54]. However, similar bactericidal and fungicidal properties have also been reported for LM EO and its main bioactive compound citral [60, 66, 67, 68, 69, 70, 71, 72]. It should be mentioned that most available antifungal drugs possess only fungistatic activity and there is an increasing fungal resistance toward these drugs.