## 3 Results
### Antimicrobial Activity  
As seen from the results presented in Table 4, the St. John's Wort EO at the concentration of 10 mg/mL exhibited the highest inhibitory activity against the Gram-negative bacteria _K. pneumoniae_ ATCC 13883 (IZ = 12.0 mm) and _P. aeruginosa_ ATCC 9027 (IZ = 11.0 mm). The antimicrobial activity against the Gram-positive bacteria _B. subtilis_ ATCC 6633, _B. cereus_ NCTC 11145, _S. aureus_ ATCC 25923, L. _monocytogenes_ NBIMCC 8632 and _E. faecalis_ ATCC 19433, the Gram-negative bacteria _S. enteritidis_ ATCC 13076 and _E. coli_ ATCC 25922, the yeasts _C. albicans_ NBIMCC 74 and _S. cerevisiae_ ATCC 9763 and the fungi _P. chrysogenum_ and _F. moniliforme_ ATCC 38932 was low. The St. John's Wort EO failed to inhibit the Gram-negative bacterium _P. vulgaris_ ATCC 6380, as well as the fungi _A. niger_ ATCC 1015 and _A. flavus_. The methanol (MeOH) used as a solvent of the EO and a control did not show antimicrobial activity against the test microorganisms used in the study.  
[PERSON] et al. [34] screened for antimicrobial activity in EOs obtained from six medicinal plants from Romania and stated that _H. perforatum_ L. EO inhibited the growth of the tested food-borne pathogens in a dose-dependent manner. The SJW EO at a dose of 10 \(\upmu\)g/disc showed inhibitory activity against _S. aureus_ ATCC 25923 (IZ = 10.24 mm), _Salmonella typhimurium_ ATCC 14028 (IZ = 9.89 mm), _E. coli_ ATCC 25922 (IZ = 9.87 mm), _K. pneumoniae_ ATCC 13882 (IZ = 11.8 mm), _E. faecalis_ ATCC 29212 (IZ = 9.79 mm) and _C. albicans_ ATCC 10231 (IZ = 22.12 mm) but did not inhibit _P. aeruginosa_ ATCC 27853. [PERSON] et al. [30] reported the efficacy of SJW EO against _S. cereus_, L. _monocytogenes_, _P. aeruginosa_ and _S. typhimurium_, with values of minimum inhibitory concentration (MIC) varying between 250 and 500 \(\upmu\)g/mL. Using the agar diffusion method, [PERSON] et al. [35] evaluated the antimicrobial activity of _H. perforatum_ EO and reference antimicrobial drugs (antibiotics) against the growth of _S. aureus_, _S. typhimurium_ and _P. aeruginosa_. The tested EO (applied singly or diluted with ethanol of olive oil) exhibited an inhibitory effect on _S. aureus_ and _P. aeruginosa_ but failed to inhibit _S. typhimurium_. The results from antimicrobial screening of six _Hypericum_ species from Serbia revealed that SJW EO was active against _B. cereus_, _Microoccus luteus_, _Sarcina lutea_, _S. aureus_ (MIC = 12.5 \(\upmu\)g/mL), _Agrobacterium tumefaciens_, _E. coli_, _Pseudomonas tolaasii_, _S. enteritidis_ (MIC = 25 \(\upmu\)g/mL), _Proteus mirabilis_, _P. aeruginosa_ and _C. albicans_ (MIC = 50 \(\upmu\)g/mL) [36].  
In addition to the inhibitory effect on some food-borne pathogens, it was also found that SJW EO was an effective antimicrobial agent against standard strains and clinical isolates of periodontal pathogens, such as _Aggregatbacter actinomycetemcomitans_ and _Por  
Figure 2: IC\({}_{50}\) of Bulgarian St John’s Wort (_H. perforatum_ L) essential oil determined by the ABTS method.  
phromoans gingialis._ The authors determined that the MIC values were between 0.312 and 0.625 ug/mL [37]. [PERSON] et al. [38] observed low antifungal activity of SJW EO against five fungal strains--_A. niger_ ATCC 6275, _A. flavus_ ATCC 9170, _Cladosporioides_ ATCC 13276, _Penicillium funciculusum_ ATCC 10509 and _Trichoderma viride_ IAM 5061. The authors determined an MIC value of 15 ug/mL against all tested strains and assumed that the most probable reason for the weak antifungal effect of the studied SJW EO was the low concentration of p-cymene (4.8%)--an aromatic monoterpene with antifungal activity. This was in agreement with the results obtained for the antifungal activity of our SJW EO, in which the concentration of p-cymene was also low (2.05%).