## 2 Results and Discussion
### Determination of the MIC and MBC of EOEP  
The antimicrobial activity of EOEP from the rhizome against foodborne microorganisms was assessed by MIC and MBC values investigated by broth dilution assay, as shown in Table 2. The results show that EOEP has a stronger antimicrobial activity against Gram-positive bacteria than Gram-negative bacteria. The MIC and MBC values against Gram-positive bacteria range between 0.02 to 0.03% (_v/v_) and 0.10 to 0.30% (_v/v_), respectively, while the MIC and MBC values against most Gram-negative bacteria and lactic acid bacteria are 5% (_v/v_) or greater. Thus, this EOEP is not strongly effective against Gram-negative bacteria. Amongst the bacteria tested, _L. monocytogenes_ strain 108 and Scott A were found to be most sensitive strains with the MIC and MBC values of 0.02% (_v/v_) and 0.10% (_v/v_), respectively, followed by _B. cereus_, which gave values of 0.02% and 0.15% (_v/v_), respectively. The antimicrobial activity of the _E. paveiana_ essential oil has been reported in few studies. There was only one report on the biological activities of the _E. paveiana_ essential oil and, although it showed cytotoxic activity on human small-cell lung cancer (NCI-H187) cells with an IC\({}_{50}\) value of 31.69 \(\upmu\)g/mL, it was inactive against _Mycobacterium tuberculosis_[18]. From our previous publication, various solvent extracts of _E. paveiana_, including ethanol, acetone, dichloromethane, ethyl acetate, petroleum ether and hexane, had antibacterial activity against Gram-positive bacteria _(Bacillus cereus, B. subtilis, Staphylococcus aureus_ and _Listeria monocytogenes_) and Gram-negative bacteria (_Escherichia coli, Pseudomonas aeruginosa, Vibrio parahaemolyticus_ and _Salmonella typhimurium_) [4]. Comparing the antimicrobial activity of the _Etlingera_ essential oils, the essential oils from _E. elalator_, _Etlingera fulgens_, _Etlingera mingayi_, _E. punicea_ and _Etlingera rubrostriatala_ had antibacterial activity against Gram-positive bacteria, namely _B. cereus_, _Microoccus luteus_ and _S. aureus_[11; 19; 20]. Moreover, the essential oil of _E. elalator_ flowers and _E. punicea_ had antifungal activity against _Candida albicans_[11; 20]. However, those essential oils were ineffective against Gram-negative bacteria, including _E. coli_, _Pseudomonas aeruginosa, Salmonella albany_ and _Salmonella_ Choleraesuis. These reports correspond to the findings we have presented above. It has been generally reported that Gram-negative bacteria are more resistant to antimicrobial agents due to the lipopolysaccharide-acrylamide layer on the outer membrane of Gram-negative bacteria being highly hydrophilic. This acts as a strong barrier against hydrophobic molecules, which are the majority of the antimicrobial agents. In contrast, the cell walls of the Gram-positive bacteria only consist of thick peptidoglycan, which make the bacteria more susceptible to attack by the antibacterial agents [13; 20; 21; 22]. Interestingly, lactic acid bacteria, which are mostly Gram-positive, are more resistant to EO than the other Gram-positive members [23; 24; 25; 26]. This effect may be due to the surface of the lactic acid bacteria, including _Lactobacillus_ spp., which are able to provide intrinsic resistance to some antibacterial agents [23].  
Figure 2: Structures of the main compounds identified in EOEP.