K Kon - Аntibacterial activity of thymus vulgaris essential oil alone and in combination with other essential oils - страница 1

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BIOSCIENCE

Vol. 4, No. 2, Pp. 50-56 July 2012

ISSN: 2087-3948 E-ISSN: 2087-3956

Antibacterial activity of Thymus vulgaris essential oil alone and in combination with other essential oils

KATERYNA KON1'*, MAHENDRA RAI2

1 Department of Microbiology, Virology, and Immunology, Kharkiv National Medical University, 61022, Pr. Lenina, 4, Kharkiv, Ukraine. Tel.

+380507174771, e-mail: katerynakon@gmail.com 2Department of Biotechnology, Sant Gadge Baba Amravati University, Amravati-444602, Maharashtra India

Manuscript received: 16 July 2012. Revision accepted: 31 July 2012.

Abstract. Kon K, Rai M. 2012. Antibacterial activity of Thymus vulgaris essential oil alone and in combination with other essential oils. Nusantara Bioscience 4: 50-56. Essential oils (EOs) from plants represent an alternative approach in combating antibiotic-resistant bacteria. One of the EOs with proven antibacterial properties is Thymus vulgaris EO. The purpose of the present work was to investigate in vitro antibacterial activity of T. vulgaris EO alone and in combination with other EOs. The activity of T. vulgaris EO was screened in combination with 34 EOs against Staphylococcus aureus and Escherichia coli by disk diffusion method; then the most effective combinations were evaluated by broth microdilution method. Against S. aureus the synergistic effect was found in combination of T. vulgaris and Cinnamomum zeylonicum EOs with fractional inhibitory concentration (FIC) index of 0.26; Juniperus communis and Picea abies EOs showed additive effect (FIC indexes were 0.74 and 0.78, respectively). Combination of T. vulgaris EO with Aniba rosaeodora and Melissa officinalis EOs demonstrated synergistic effect against E. coli (FIC indexes were 0.23 and 0.34, respectively); combination of T. vulgaris and Mentha piperita EOs was additive (FIC index 0.55). Therefore, combining T. vulgaris EO with other EOs has potential in further enhancing its antibacterial properties.

Key words: Thymus vulgaris, essential oils, combinations, Staphylococcus aureus, Escherichia coli.

Abstrak Kon K, Rai M. 2012. Aktivitas antibakteri minyak atsiri Thymus vulgaris tunggal atau campuran dengan minyak atsiri lain. Bioscience Nusantara 4: 50-56. Minyak atsiri tumbuhan merupakan senyawa alternatif untuk melawan bakteri resisten antibiotik. Salah satu minyak atsiri yang terbukti bersifat antibakteri adalah minyak atsiri Thymus vulgaris. Penelitian ini bertujuan untuk mengetahui aktivitas in vitro antibakteri minyak atsiri T. vulgaris tunggal atau campuran dengan minyak atsiri lain. Aktivitas antibakteri minyak atsiri T. vulgaris dan campurannya dengan 34 minyak atsiri lain terhadap Staphylococcus aureus dan Escherichia coli ditapis dengan metode cawan difusi, kemudian campuran yang paling efektif diuji dengan metode mikrodilusi kaldu. Efek sinergis terhadap S. aureus ditemukan pada campuran antara minyak atsiri T. vulgaris dan Cinnamomum zeylonicum dengan indeks konsentrasi hambat fraksional (FIC) 0,26; minyak atsiri Juniperus communis dan Picea abies menunjukkan efek aditif (indeks FIC masing-masing adalah 0,74 dan 0,78). Campuran minyak atsiri T. vulgaris dengan Aniba rosaeodora dan Melissa officinalis menunjukkan efek sinergis terhadap E. coli (indeks FIC masing-masing adalah 0,23 dan 0,34); campuran minyak atsiri T. vulgaris dengan Mentha piperita menunjukkan efek aditif (indeks FIC 0,55). Oleh karena itu, campuran minyak atsiri T. vulgaris dengan minyak atsiri lainnya memiliki potensi untuk meningkatkan sifat antibakteri.

Kata kunci: Timus vulgaris, minyak atsiri, kombinasi, Staphylococcus aureus, Escherichia coli

INTRODUCTION

Wide spread of antibiotic resistance remains a serious clinical problem, which stimulates studies for search of new methods for coping with drug resistance or renews interest in traditionally used and forgotten methods, such as treatment with antibacterial plant extracts and essential oils (EOs) (Rios and Recio 2005; Fisher and Phillips 2009). Combined therapy is traditionally used to increase antimicrobial activity and reduce toxic effects of agents (Houghton 2009).

Thyme plant is used since ancient times to achieve healing, antiseptic fumigator, food preservation and other useful effects (Stahl-Biskup and Saez 2002). Nowadays, Thymus vulgaris EO belongs to EOs with the most pronounced antimicrobial activity (Iten et al. 2009). It was shown to be active against many bacteria, viruses and fungi. High antimicrobial activity of thyme oil and its components, first of all thymol and carvacrol, was demonstrated against Staphylococcus aureus (Al-Bayati 2008; Sokovic et al. 2010; Levic et al. 2011), including methicillin-resistant isolates (Tohidpour et al. 2010), S. epidermidis (Sokovic et al. 2010), Enterococcus faecalis (Levic et al. 2011), Bacillus cereus (Al-Bayati, 2008), Vibrio cholerae (Rattanachaikunsopon and Phumkhachorn, 2010), Escherichia coli (Levic et al. 2011), Proteus mirabilis (Sokovic et al. 2010; Levic et al. 2011), P. vulgaris (Al-Bayati, 2008), Pseudomonas aeruginosa (Sokovic et al. 2010), Salmonella enteritidis (Sokovic et al. 2010), S. choleraesuis (Levic et al. 2011), S. typhimurium

 (Sokovic et al. 2010), and other microorganisms.

In spite of many studies devoted to thyme oil, its combinations with other EOs have not been paid much attention. Gutierrez et al. (2009) studied combinations composed of thyme and oregano EOs against B. cereus, E. coli, Listeria monocytogenes and P. aeruginosa by checkerboard method and found that thyme-oregano EO combination had additive effect against B. cereus and P. aeruginosa, and indifferent effect against E. coli and L. monocytogenes. Furthermore, against L. monocytogenes the authors studied five more thyme EO combinations - with basil, lemon balm, marjoram, rosemary, and sage EOs. The results showed that basil, rosemary and sage EOs with thyme oil had additive effect, while lemon balm and marjoram EOs were indifferent.

The analysis of available literature shows that EO combinations, including combinations with thyme EO, represent perspective approach in antimicrobial treatment and prevention, however, in contrast to combinations of traditional antibiotics, this topic is not still well studied and requires further investigations.

The main goal of the present study was to investigate antimicrobial activity of thyme EO in combination with different EOs against S. aureus and E. coli.

MATERIALS AND METHODS

Essential oils. We used commercial EO of Thymus vulgaris (purchased from NPF Zarstvo Aromatov, Sudak, Ukraine) and 34 different EOs (purchased from Aroma Inter, Mykolaiv, Ukraine; Aromatika, Kiyiv, Ukraine; NPF Zarstvo Aromatov, Sudak, Ukraine) (Table 1).

Strains, preparation of inocula. We used reference strains Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922). The cultures of bacteria were maintained in meat peptone agar slants at 4°C throughout the study and used as stock cultures. For preparation of inocula, cultures were grown until logarithmic phase, and then bacterial density was adjusted to approximately 108 colony forming units (CFU) per mL for disk diffusion method and 105 CFU/mL for microdilution method with sterile saline solution. Bacterial counts were confirmed by plating out on meat-peptone agar, plates were incubated at 37°C for 24 h.

Disk diffusion method. This method was used for the screening of EOs for increase of antibacterial activity in the presence of thyme oil. Bacterial suspension was spread over the plates 85 mm in diameter containing Mueller-Hinton agar using a sterile cotton swab in three directions in order to get a uniform microbial growth. Under aseptic conditions empty sterile disks were impregnated with 5 ці of EO. Disks were left for 5 min at room temperature for better oil absorption and were then placed on inoculated agar surface. A standard disc with ciprofloxacin (10 |ig/disc) was used as a reference control. The Petri dishes were left for 30 min at room temperature (20-22°C) for better oil diffusion and were then placed to an incubator at 37°C for 24 h. After an incubation period diameters of inhibition zones around the disks with EOs were measured.

We assessed diameter of inhibition zones around the disks with EOs mixtures. For this purpose, we prepared blends of EOs in sterile eppendorf tubes by mixing 50 ці of thyme oil with 50 ці of correspondent second oil. Paper disks were then impregnated with 5 ці of appropriate mixture of EOs. Results of disk diffusion assay for study of EO mixture were assessed by comparing the experimental inhibition zone area of oils mixture with theoretical inhibition zone area of indifferent combinatory effect (calculated as V of inhibition zone area for thyme oil + V of inhibition zone area for the second oil).

Minimal inhibitory concentration (MIC) test. We prepared serial doubling dilutions of each plant EO in 96-well microtiter plates in volume 50 of Muellor Hinton Broth to give a range of concentrations from 0.0025% to 5% (volume/volume). After preparations of suspension of tested cultures 50 were added to oil dilutions to produce total volume of 100 The resulting suspensions were then mixed with a micro-pipettor. Two controls were used: positive (50 of medium and 50 of culture), and negative (100 of medium). All microtiter plates with microorganisms were incubated at 37°C for 24 h. Inhibition of bacterial growth in the wells containing test oil was judged by comparison with growth in negative control well. The MICs were determined by measuring optical density at 570 nm and defined as the concentration of oil at which there was a sharp decline in the absorbance value.

MICs determination of mixtures of EOs. Mixture of thyme and different EOs in ratios 1:1 were tested for determinations of MICs by broth microdilution method.

In order to assess results of MICs of EOs in mixtures we calculated fractional inhibitory concentrations (FIC) with FIC indexes (Houghton 2005). Because mixtures were used in ratio 1:1, individual MIC of EO in blend was calculated as V of MIC of blend. Accordingly to this, FIC indexes were calculated as the following:

FIC of thyme oil = (1/2 MIC of blend)/ (MIC of thyme oil alone);

FIC of second oils = (1/2 MIC of blend)/ (MIC of second oil alone);

FIC index = (FIC of thyme oil) + (FIC of second oil),

where second oil is the EO which was tested in combination with thyme oil.

FIC indexes were interpreted as following: synergy, FIC < 0.5; addition, 0.5<FIC<1; indifference, 1<FIC<4; antagonism, FIC>4 (Gutierrez et al. 2009).

Chemical composition. The main components of EOs were identified by mass-spectrometry analysis. The relative amount of individual components of the total oil was expressed as percentage peak area relative to total peak area. Qualitative identification of the constituents was performed by comparison of their relative retention times and mass spectra with those stored in NIST library or with mass spectra from literature (Stein et al., 2002).

Statistical analysis of data. All experiments were repeated in triplicates, and then mean values for diameters of inhibition zones, geometric mean MICs and accordingly to them FICs were calculated. Results were analysed using statistical software SPSS (version 20.0). The results are expressed as mean value ± standard deviation or as

geometric mean. Comparison of groups was performed by U test Mann-Whitney and Kruskal-Wallis 1-way analysis of variance (ANOVA); differences were considered as statistically significant at p<0.05.

RESULTS AND DISCUSSION

Antibacterial activity of essential oils alone

The antibacterial activity of thyme oil and 34 EOs is summarized in the Table 1. The results proved that thyme EO had significant activity against S. aureus and E. coli with diameters of inhibition zones 22.74±1.56 mm and 22.46±5.48 mm, respectively. Furthermore, the majority of EOs possessed antimicrobial activity, but in very wide ranges. In general, activity of EOs was higher against S. aureus than against E. coli.

Multivariate analysis showed presence of significant differences between activity of EOs from different plant families (p=0.036). The highest activity against both tested strains was demonstrated by EOs of plants from Lamiaceae family with the mean inhibition zone 21.7±17.0 mm against S. aureus and 13.2±10.3 mm against E. coli. Rather high activity was also present in Lauraceae plant EOs against S. aureus (13.7±10.0 mm) and Myrtaceae plant EOs against E. coli (12.4±6.2 mm). Activity of Pinaceae and Rutaceae plant EOs against both strains was rather low.

S. aureus did not show any sensitivity to two EOs -eucalyptus and lemon. We found weak activity in juniper berry, rosemary, silver fir, grapefruit, pontica wormwood, and camphor white EOs. High antistaphylococcal activity was found in lavender, ylang-ylang, clary sage, clove, cedarwood, geranium, and especially in cinnamon EO.

Against E. coli total absence of activity was noticed in eight EOs: calamus, camphor white, cedarwood, juniper berry, patchouli, sandalwood, Satsuma mandarin, and silver fir. Seven more EOs showed very weak antimicrobial activity with diameter of inhibition zone not exceeding 7 mm: thuja, bitter orange, grapefruit, lime, bay laurel, ylang-ylang and dill. Interestingly, among these EOs without antimicrobial effect against E. coli some EOs possessed high activity against S. aureus, such as cedarwood, which did not inhibit growth of E. coli but had inhibition zone against S. aureus 28.4±14.1 mm; ylang-ylang EO had inhibition zones 7.0±0.9 mm against E. coli and 21.7±8.0 mm against S. aureus; patchouli and sandalwood EOs also did not inhibit growth of E. coli but had inhibition zones against S. aureus 16.9±2.8 mm and 15.3±5.1 mm, respectively.

Along with high activity of thyme EO against E. coli, high sensitivity of this strain was also shown only to two more EOs - clove and cinnamon (diameters of inhibition zones were 22.0±1.8 mm and 37.4±4.0 mm, respectively). Moderate level of activity against E. coli was demonstrated by lemon balm, peppermint and tea tree EOs with diameters of inhibition zones 10.4±1.3 mm, 10.8±1.3 mm, and 15.0±1.6 mm, respectively.

Twenty one of 35 studied EOs had significant differences in antibacterial activity against S. aureus and E.

coli, and 17 of these oils (basil, clary sage, lavender, patchouli, bay laurel, camphor white, cedarwood, silver fir, bitter orange, lime, Satsuma mandarin, calamus, dill, geranium, sandalwood, thuja, and ylang-ylang) had higher activity against S. aureus. Interestingly, peppermint, eucalyptus, tea tree and lemon EOs were more active against E. coli. Such differences in spectrum of antibacterial activity may be a good basis for further assessment of combinations between EOs.

Antibacterial activity of essential oils in combination with thyme oil: results of disk diffusion method

EOs exhibited wide range of interaction effects with thyme oil from strong antagonism to strong synergism against both tested strains. In general, enhancing effect with thyme EO was more noticeable against S. aureus than against E. coli: mean change of inhibition zone areas compared with theoretical area of indifferent interaction was (32.3±60.0)% against S. aureus, while against E. coli it was (-13.5±42.5)% (p < 0.001). Therefore, against S. aureus, in general, interactions between thyme and other EOs were synergistic, while against E. coli - antagonistic.

Compared with EOs alone, in combination with thyme oil a smaller number of EOs demonstrated significant differences in activity against tested strains: 14 EOs (basil, clary sage, lemon balm, patchouli, cedarwood, clove, siberian cedar, neroli, Satsuma mandarin, geranium, pontica wormwood, sandalwood, thuja, and ylang-ylang) were significantly more active against S. aureus than against E. coli. EOs, which alone were significantly more active against E. coli (peppermint, eucalyptus, tea tree and lemon), in combination with thyme oil demonstrated equal activity against both strains.

Against S. aureus the highest level of enhancing effect by using disk diffusion method was detected in Norway spruce EO: diameter of zone inhibition was changed from 8.6±1.5 mm without thyme oil to 32.1±13.7 mm in the mixture with thyme oil. Therefore, area of inhibition zone of mixture of thyme and Norway spruce oils was bigger than theoretical area of indifferent combination by 275.4%. High enhancing effect with thyme oil was also characteristic for juniper berry EO (Figure 1). Interestingly, that with almost absent antibacterial activity alone, in combination with thyme oil inhibition zone area increased by 145.1% compared with theoretical area of indifferent interaction. Significant enhancing effect with thyme oil was also demonstrated by thuja oil (inhibition zone area increased by 95.2%), clove (93.5%), cinnamon (77.0%), and Siberian cedar EOs (76.2%). It is worth to mention that eucalyptus and lemon EOs, which did not show antibacterial activity, in combination with thyme oil demonstrated noticeable increase in inhibition zone areas -by 55.1% and 56.1% respectively. Near 50% increase in inhibition areas was also found in lavender and lemon balm oils combined with thyme EO. Among 34 studied EOs 9 had antagonistic interactions with thyme oil: bay laurel, bitter orange, peppermint, camphor white, patchouli, silver fir, myrtle, rosemary, and especially calamus EO.

Table 1. Diameters of inhibition zones of essential oils alone and in mixture with thyme oil

English name

Lamiaceae

Basil

Cinnamon

Clary sage

Lavender

Lemon balm

Patchouli

Peppermint

Rosemary

Thyme

Lauraceae

Bay laurel

Camphor white

Cedarwood

Rosewood

Myrtaceae

Cajuput

Clove

Eucalyptus

Myrtle

Tea tree

Pinaceae

Norway spruce

Siberian cedar

Silver fir

Rutaceae

Bitter orange

Grapefruit

Lemon

Lime

Neroli

Satsuma mandarin Other

Calamus

Dill

Geranium Juniper berry Pontica wormwood Sandalwood Thuja

Ylang-ylang Control

Ciprofloxacin

Essential oils

Latin name

Ocimum basilicum Cinnamomum zeylonicum Salvia sclarea Lavandula anqustifolia Melissa officinalis Pogostemon patchoufy Mentha piperita Rosmarinus officinalis Thymus vulgaris

Laurus nobilis Cinnamomum camphora Juniperus virginiana Aniba rosaeodora

Melaleuca cajeputi Eugenia caryophyllata Evcalyptus globulus Myrtus communis Melaleuca alternifolia

Picea abies Pinus sibirica Abies sibirica

Citrus aurantium (fruits) Citrus paradisi Citrus limon Citrus aurantifolia C. aurantium (flowers) Citrus unshiu

Diameter of inhibition zone alone (Mean±SD)

Diameter of inhibition zone in combination with thyme oil (Mean±SD)

S. aureus    E. coli     p   S. aureus    E. coli

21.7±17.0

15.8±3.0

64.2±2.3

23.3±6.7

21.5±19.5

16.4±8.2

16.9±2.8

7.7±0.5

7.0±0.3

22.7±1.6

13.7±10.0

10.9±1.6

7.4±0.4

28.4±14.1

7.9±0.8

10.8±7.7

7.7±0.9

24.6±7.2

6.0±0.0

7.8±2.1

8±1.0

8.2±1.0

8.6±1.5

8.9±0.9

7.1±0.7

8.3±1.8

8.2±0.1

7.2±0.2

6.0±0.0

10.0±1.2

10.6±2.9

7.7±0.5

Acorus calamus (Araceae) 13.1±3.3

Anethum graveolens (Apiaceae) 9.1±0.7

Pelargonium roseum (Geraniaceae) 29.2±5.6

Juniperus communis (Cupressaceae) 6. 7±0. 6

Artemisiapontica (Asteraceae) 7.3±0.6

Santalum album (Santalaceae) 15.3±5.1

Thuja occidentalis (Cupressaceae) 9.7±1.6

Cananga odorata (Annonaceae) 21.7±8.0

13.2±10.3

8.9±0.6

37.4±4.0

8.4±0.4

7.2±0.1

10.4±1.3

10.8±1.3

7.4±0.5

22.5±5.5

7.1±1.6

6.9±0.5

9.5±3.4

12.4±6.2

8.1±0.6

22.0±1.8

9.5±0.7

7.3±0.4

15.0±1.6

6.8±0.7

7.5±1.1

7.4±1.4

7.4±1.5 6.6±1.0 6.6±1.0 8.8±0.5 6.8±0.7 9.8±1.0

7.0±0.8 8.3±0.5

7.9±0.6

6.5±0.8 7.0±0.9

0.15 0.05 0.08 0.05 0.05

0.13 0.04 0.05 0.28 0.85 0.08 0.05 0.03 0.03 0.51 0.35 0.28 0.83 0.04 0.83 0.05 0.13 0.48 0.28 0.04 0.30 0.05 0.51 0.04 0.05 0.83 0.04

0.04 0.05 0.05 0.12 0.27 0.04 0.05 0.05

7±16.0 3±5.6 4±6.6 4±1.0 7±15.6 0±4.9 3±1.8 5±6.4 6±3.5

0±6.6 6±3.5 2±4.2 6±8.8 4±3.4 0±6.6 4±2.9 3±6.4 9±4.6 4±5.7 0±4.3 7±8.4 1±13.7 7±4.9 3±2.1 0±1.3 9±1.0 0±4.9 0±5.3 9±3.0 1±1.5 9±0.3

1±2.7 4±5.1 7±2.5 3±4.6 2±3.0 6±8.1 2±1.2 7±6.9

16.7±6.3 9.4±1.0 29.9±6.9 16.1±0.9 12.3±0.8 18.6±1.8 12.4±1.2 18.9±1.0 15.7±2.3

17.8±5.3 16.2±3.1 17.4±5.9 12.6±1.0 25.2±5.2 16.3±1.8 14.7±1.2 16.7±2.9 17.9±0.2 14.1±1.9 17.9±0.2 15.0±1.2 16.1±0.5 15.1±1.5 13.7±1.6 15.7±1.8 16.4±0.3 12.7±2.8 18.3±0.5 15.8±2.3 15.8±2.2 15.2±4.2

9.3±2.5 16.8±3.6 14.1±1.2 20.4±5.9 13.5±0.8 10.5±0.6 13.7±1.1 11.5±1.6

P_

0.07

0.05 0.08 0.05 0.13 0.05 0.05 0.51 0.83

0.25 0.83 0.83 0.05 0.28 0.12 0.13 0.05 0.51 0.83 0.51 0.13 0.13 0.05 0.51 0.01 0.83 0.28 0.51 0.28 0.05 0.05

0.28 0.83 0.05 0.28 0.05 0.05 0.05 0.05

Fold increase (%) of inhibition area comparing with theoretical area of

indifference S. aureus   E. coli

2.3% 77.0% 27.0% 50.1% 50.7%

-20.8% -12.0% -29.8%

-8.5% -14.8%

36.9% 21.5%

-2.0% 93.5%

55.1%*

-23.4% 47.6%

275.4% 76.2% -22.9%

-8.7% 5.4% 56.1%* 8.5% 20.3% 28.3%

-53.4% 5.3% -1.0% 145.1% 46.2% 31.6% 95.2% 38.4%

-54.9% -9.7% 36.4% -60.6% 65.9% -38.8%* 67.5% -35.5%

1.3% -19.1%* -37.2%* 128.6%

-44.3% -47.3% -4.5% -23.9% -20.2%

-17.2% 26.1% -49.4%*

5.8% -36.5% 2.3% -3.0% -11.4% -8.2%*

-76.8%* 9.6% -48.9% 12.0%* -2.0% -64.4%* -50.2% -48.9%

28.8±1.7 38.7±0.2

Note: * In the absence of bacterial growth inhibition zones, the disks' diameters (6 mm) were used to calculate fold increase, %

2.A

2.B

Figure 1. Inhibition zones around the disk with juniper berry essential oil alone (left) and mixture of juniper berry and thyme essential oils (right) (A); inhibition zone around the disk with thyme essential oil alone (B) against Staphylococcus aureus

Figure 2. Inhibition zones around the disk with rosewood essential oil alone (left) and mixture of rosewood and thyme essential oils (right) (A); inhibition zone around the disk with thyme essential oil alone (B) against Escherichia coli

Against E. coli rosewood EO showed significant enhancing effect in combination with thyme oil (Figure 2) - inhibition zone area increased by 128.6% compared with theoretical area of indifferent interaction. High enhancing effect with thyme oil was also demonstrated by peppermint and lemon balm EOs: zones of inhibition increased by 67.5% and 65.9%, respectively. Several more EOs (clary sage, Siberian cedar, juniper berry, dill, and bitter orange) had some enhancing effect in ranges from 36.4% for clary sage to 5.8% for bitter orange EO. Eucalyptus, lime, pontica wormwood, bay laurel and lemon EOs were indifferent to the presence of thyme oil, while majority of EOs (21 of 34) exhibited antagonistic interactions with thyme oil from mild (decrease of inhibition zone by 9.7% for cinnamon oil) to strong antagonism in lavender, sandalwood and calamus EOs (zones of inhibition decreased by 60.6%, 64.4%, and 76.8%, respectively). Interestingly, that calamus EO showed significant antagonistic effect with thyme oil against both tested strains, furthermore, antagonism was more noticeable against E. coli: decrease of inhibition zone area was 76.8% against E. coli and 53.4% against S. aureus.

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K Kon - Аntibacterial activity of thymus vulgaris essential oil alone and in combination with other essential oils