Composition and evaluation of the antimicrobial activity of the essential oil of Senecio selloi Spreng DC

The essential oil of the aerial parts of Senecio selloi Spreng. DC. was extracted by hydrodistillation and analyzed by GC/MS. Nineteen compounds were identified, representing 99.9% of the total. The main compounds were found to be sesquiterpene hydrocarbons (71.3%), most of them with a bisabolane skeleton (59.4%). The major constituent was α-zingiberene (54%), followed by monoterpene α-isolimonene (16%). The essential oil was also tested against two Gram-positive and two Gram-negative bacterial species, three yeasts, and an algae. From the strains assayed, only Bacillus subtilis ATCC 6633 showed susceptibility (MIC and MBC = 4400 μg/mL) to the essential oil.


INTRoDuCTIoN
According to Bolzan et al (2007), 46 Latin American Senecio species are used for therapeutic purposes, including the treatment of fungal skin infections (Portillo et al., 2001), pneumonia (Hammond et al., 1998) and antiseptic (Bah et al., 1994).This information suggests antifungal and/or antibacterial activities for preparations obtained from this species.
The genus Senecio (Asteraceae) is constituted of about 2000 species worldwide, 85 of which are found in southern Brazil and of these, 33 are native of the state of Rio Grande do Sul (Cabrera & Klein, 1975).Despite the large number of species and its classification in a botanical family which is rich in essential oils, there are only few reports on the composition and biological activities of this group of compounds for the genus.
Essential oils are a mixture of volatile components with different biological functions, which are necessary for the survival of the produced plant species.They often play a crucial role in the defense against microorganisms (Siqui et al., 2000).The essential oils have been used for bactericidal, virucidal, fungicidal, antiparasitic, insecticidal, medicinal and cosmetic applications, yet in pharmaceutical, sanitary, cosmetic, agricultural and food industries (Bakkali et al., 2008).
This paper reports the chemical analysis of the essential oil obtained from aerial parts of Senecio selloi Spreng.DC. and its antimicrobial evaluation.

Plant material
Aerial parts of Senecio selloi Spreng.DC.

Essential oil extraction and analysis
The essential oil was obtained from the fresh aerial parts (2.507 kg) by hydrodistillation using Clevenger type apparatus for 2 h (Farmacopéia Brasileira, 2000) and then stored at -4 ºC in amber glass bottles.Its yield was calculated as the ratio between the weight of the fresh plant and weight of the essential oil obtained (% w/w).
The constituents of the essential oils were identified based on retention index (RI), determined using a calibration curve of a homologous series of n-alkanes (C 8 -C 32 ) injected under the same chromatographic conditions of samples and models fragmentation of mass spectra, both compared with literature data.(Adams, 2001) (Table 1).
The concentration of constituents was calculated using the full area of their peaks, related to the total area of all the constituent sample, obtained by analysis using gas chromatograph.

Antimicrobial activity
The microorganisms used in the test are cataloged strains and clinical isolates, and they are listed in Table 2.The antimicrobial activity of the essential oil was determined by the broth microdilution method (NCCLS, 1997;NCCLS, 2002).Initially, the essential oil was dissolved in methanol in order to obtain a stock solution which was diluted (1:100) in the appropriate medium; Mueller-Hinton broth for bacterial strains and buffered RPMI 1640 broth for yeasts.From these, serial dilutions were prepared in 96-well microtiter trays.
Bacterial strains and yeasts were grown in Mueller-Hinton (24 h / 35 ºC) and Sabouraud dextrose agar (48 h / 35 ºC), respectively.Bacterial suspensions of 0.5 McFarland standard turbidity (1 x 10 8 CFU/mL) were prepared by dilution with saline, and later diluted (1:100) with Mueller-Hinton broth (1 x 10 6 CFU/mL).Aliquots of 10 mL (1 x 10 4 CFU/mL) of inoculum were added to each well of the microtiter tray already containing 200 mL of the medium plus different concentrations of the essential oil.In parallel, yeast suspensions were prepared by dilution with 0.85% NaCl solution and adjusted to 0.5 McFarland standard turbidity (1 x 10 6 to 5 x 10 6 CFU/ mL).These suspensions were diluted with sterilized water (1:50) and buffered RPMI 1640 broth (1:20), resulting in 1.0 x 10 3 to 5.0 x 10 3 CFU/mL.100 µL of the inoculum was added to each well of the microtiter plate already containing 100 µL of the essential oil solution in different concentration.The microplates with bacterial strains were incubated for 24 h at 35 °C.Those with yeasts were incubated for 48 h at 35 °C.The same tests were performed simultaneously for inoculum growth and sterility control.The final concentration of methanol did not exceed 1% and did not affect the growth of the microorganisms.The algae Prototeca zopfii was assayed in the same way as yeasts were described.
The minimum inhibitory concentration (MIC) was determined as the lowest concentration of essential oil that completely inhibited visible growth of microorganisms.To determine the minimum bactericidal and fungicidal concentration (MBC/ MFC), 10 mL broth was taken from each well without microbial growth and inoculated in Mueller-Hinton and Sabouraud dextrose agar, respectively.The plates were incubated for 48 h at 35 °C until they were read.MBC and MFC were defined as the lowest concentration of the essential oil capable of killing the inoculum.

RESuLTS AND DISCuSSIoN:
The essential oil of the fresh aerial parts of S. selloi gave a yield of 0.0035%, which can be considered low if compared with the yields obtained for the essential oils of the aerial parts of other Senecio species (Francescato, 2007).Results of the chemical analysis of the essential oil are shown in Table 1.In the oil, we observe the prevalence of sesquiterpenes, fact that was also reported for the essential oil of several species of Senecio (Francescato, 2007;Murari et al., 2008)   the sesquiterpenes, the derivatives of bisabolane skeleton predominate with 59.4% of the composition of oil, being a-zingiberene the major component, with 54%.
Results concerning the evaluation of the antimicrobial activity are shown in Table 2.
The essential oil of the aerial parts of S. selloi presented activity only against one Grampositive strain, Bacillus subtilis ATCC 6633, with values for MIC and MBC of 4400 mg/mL.B. subtilis is a microorganism of interest because of its wide industrial application, its similarity to pathogenic strains, and its role as a model organism for Grampositive sporulating bacteria (Henry et al., 2009).
The absence of susceptibility of Gramnegative bacteria against the essential oil evaluated is not surprising because of the dual membrane presented by these microorganisms, which prevents the penetration of various antimicrobial agents.Additionally, their periplasmic space contains enzymes that are capable of breaking down foreign molecules introduced into the environment (Duffy & Power, 2001).
Different authors attribute the ability of essential oils to inhibit the growth of microorganisms to the hydrophobicity of their constituents.This feature would allow their partition in the bilipid layer cell membrane, increasing permeability and leading to loss of vital cell content (Burt, 2004;Juven et al., 1994;Kim et al., 1995).However, other studies have reported the relevance of an oxygen function in the molecule for this activity.According to Dorman & Deans (2000), alcohols have recognized bactericidal activity against vegetative cells, acting as protein denaturing agents, solvents or dehydrating agents.Other authors have reported that components of essential oils containing phenolic or alcoholic groups exhibit more pronounced inhibitory effects on microbial growth, followed by aldehydes and ketones (Griffin et al., 1999).When hydrophobicity is crucial for the antimicrobial activity, a good antimicrobial activity for the essential oil of S. selloi would be expected, since the major constituents are hydrocarbons.In contrast, the antibacterial activity was detected only for a single strain of Gram-positive bacteria and only in high concentrations.
Some peculiarities of essential oils such as volatility, low solubility in water and their complexity, significantly interfere with the results of tests for evaluation of antimicrobial activity.Other important factors to take into consideration are the characteristics of each microorganism and the oil to be analyzed.In experiments with fungi, for example, we must take into account the incubation time of 48 hours, which may interfere with the results due to the decomposition or evaporation of oil during the test period (Nascimento et al., 2007).
The absence of oxygenated compounds in the oil is another likely explanation for the high values of MIC and MBC detected in this study (Murari et al., 2008).Therefore, the results described in this work reinforce the importance of the oxygenated function to the antimicrobial activity of essential oils.Although we did not evaluate the antimicrobial activity of the essential oil of S. selloi against phytopathogens, the results obtained suggest that its components do not act in plant defense against microorganisms, however, it must have other biological functions.In fact, the literature describes different biological activities for its major constituent, zingiberene, which shows insecticide, repellent and antifeedant activities (Antonious & Kochhar, 2003).

TABLE 1 .
(Adams, 2001of the essential oil of the aerial parts of S. selloi.KI = Literature Kovats index(Adams, 2001) KI C = calculated Kovats index UC: Unidentified Compound

TABLE 2 .
Minimum inhibitory concentration values of the essential oil of Senecio selloi against bacteria, yeasts and algae.