Infectious diseases are common reasons of morbidity and mortality in the world . Introduction of antibiotics have had a consequence not only on the management of infections but also on society by changing morbidity and mortality . However, the abuse of these compounds has led to the emergence and increasing of multidrug-resistant pathogens . The situation is deteriorated by the increasing the number of antibiotic-resistant pathogens and potential to endure after exposure to antimicrobial agents . As no new drugs have been introduced to manage antibiotic-resistant pathogens, and as it seems doubtful that any novel agents will be established presently, clinicians may become obliged to administrate some drugs regardless of their complications . Hence, antibiotic-resistant pathogens are public health crisis and the need to explore and identify new compounds with antibacterial properties without toxic effects on human cells is obvious .
Plants are one source of the compounds with antimicrobial activity that provides options of novel alternative drugs for microbial disease . Essential oils derived from plants are one of the most important agricultural products with antimicrobial property . About 3000 essential oils produced by at least 2000 plant species, which about 300 of them are significant from the marketing viewpoint . Essential oils and their constituent small molecules exhibit excellent medicinal properties and hence may be used against infectious and noninfectious diseases . Essential oils are definite as any volatile oil(s) that have strong aromatic components and that give characteristic odor, flavor, or smell to a plant. These are the byproducts of plant metabolism and are frequently referred to as volatile plant secondary metabolites. Essential oils are found in glandular hairs or secretory cavities of plant-cell wall and are present as droplets of juice in the leaves, stems, bark, flowers, roots, and/or fruits in different plants . Carvacrol and thymol are the major constituents of the essential oils, which belong to the Lamiaceae family of plants including oregano and thyme . In this study, we review antimicrobial effects of carvacrol and thymol.
The ancient Egyptians used thymol and carvacrol as protective agents to preserve the mummies . They were also used as an active additive in food flavoring, perfumes, cosmetics, mouthwash, and some of them have been made for massaging the joints and to treat nail fungi as topical ointments. Drugs formulated from these compounds were administered to care for infections of the mouth and throat and prevent of gingivitis .
Thymol (also known as 2-isopropyl-5-methylphenol) (Fig. 1), a phenolic compound present in essential oils, is a natural monoterpene and carvacrol isomer that extracted from thyme and the other kinds of plants . Thymol is less water soluble at neutral pH, but it is as well soluble in some organic solvents and alcohols . It has been observed its antioxidant, antispasmodic, antimicrobial, and anti-inflammatory property . It is a p-cymene derivative compound and is also identified for the antiseptic and antimicrobial effects . Some studies have reported the usage of thymol for anticancer property . The antioxidant effects of thymol and carvacrol have been confirmed in several studies, suggesting their administration as nutritious elements in the improvement of novel functional foods . Thymol protective nature against caries and plaques allures the field of dental drugs .
Carvacrol (5-isopropyl-2-methylphenol), (Fig. 1), is also monoterpene that found in many plant species such as thyme and with greater amount in oregano . Carvacrol is significant component of essential oils and recently has attracted much attention as a result of its biological properties, such as a wide spectrum of antimicrobial activity. Because carvacrol exhibits strong antioxidative properties and both hydrophobic properties associated with the substituted aromatic ring and hydrophilic properties associated with the phenolic OH group, numerous studies report its antioxidative, anti-inflammatory, antibacterial, antifungal, antiprotozoal, anticarcinogenic, antidiabetic, antinociceptive, cardioprotective, and neuroprotective properties .
Antibacterial effect of thymol and carvacrol
Several studies were reported antibacterial effects of thymol alone or in combination with other substance such as carvacrol . These compounds can inhibit growth of both gram-positive and gram-negative bacteria . Low toxicity and pleasant smell as well as taste of thymol show that this material can be used as an additive to prevent bacterial spoilage . Trombetta et al.  report the antimicrobial efficacy of thymol against Staphylococcus aureus and Escherichia coli. Some researchers speculated that the antibacterial mechanism of thymol may consequence, at least partly, from a perturbation of the lipid fraction of the bacterial plasma membrane, resulting in changes of membrane permeability and in the escape of intracellular content [27,28]. Lambert, et al. exhibited antibacterial effect of thymol and carvacrol against Pseudomonas aeruginosa and S. aureus as a result of disruption in membrane integrity, which further affects the pH homeostasis and balance of inorganic ions . Therefore, antibacterial property of carvacrol and thymol is dependent to their capability to permeabilize, depolarize, and disruption of the cytoplasmic membrane.
Gas chromatographic mass spectrometric examination indicated thymol is major essential oil of Monarda punctata. The results of study carried by Li et al.  indicated that Streptococcus pyogenes, E. coli, and Streptococcus pneumonia were the most susceptible to thymol, whereas methicillin-resistant S. aureus was reported to be the most resistant to the essential oil with relatively higher Minimum Inhibitory Concentration (MIC) and Minimum bactericidal concentration (MBC) values. The disk diffusion method data show thymol is most effective against Brochothrix thermosphacta (Inhibition Zone: 39.7 mm) followed by Listeria monocytogenes and Salmonella thyphimurium (Inhibition Zone: 35.6 and 33.3 mm, respectively). The MIC and MBC values (0.25 and 0.5 μg/ml, respectively) were the same for L. monocytogenes, S. thyphimurium, and E. coli O157:H7. Pseudomonas fluorescens was the least inhibited by thymol (MIC and MBC ranging from 1 to 1.5 μg/ml). These components could be probable options to be applied as natural alternatives for further usage in food conservation to hold up or inhibit the bacterial increase and for protection and to expand the shelf existence of the food products. However, the verification of antibacterial effects and organoleptic impact of these essential oils in foodstuffs require assessing . Results of several studies were confirmed bactericidal effects of thymol and carvacrol against of pathogens and food spoilage bacteria (Table 1) [24,31–39].
The antibacterial efficacy of carvacrol and thymol in combination with other antibacterial compounds on gram-negative and gram-positive organism were evaluated in some studies. The results of these studies will be affected by the methods for detection of synergy effects. For example Hamoud et al. , reported checkerboard data indicate indifferent interaction against gram-positive and synergy against gram-negative bacteria, whereas time-kill analyses advocate synergistic achievement in diverse combinations against both types of bacteria. Combinations of thymol and carvacrol with antibacterial (azithhromycin, clarithromycin, minocycline, and tigecycline) using checkerboard indicted achievement a synergism in the great majority of cases . Thymol and carvacrol were found to be highly efficient in increasing the susceptibility of S. typhimurium to ampicillin, tetracycline, penicillin, bacitracin, erythromycin, and novobiocin and resistance of S. pyogenes to erythromycin . On the basis of these data, the authors recommended that thymol in combination with specific antimicrobial drugs may be an efficient alternative option to treat infections.
Effect of thymol and carvacrol on biofilm formation
Biofilm biomass is a mixture of exopolysaccharides, proteins, DNA, and extracellular matrix that has the stabilizing role of biofilm construction . Bacteria in a biofilm are much more resistant to antibiotics than to planktonic status . The plant derivatives can effect on microbial biofilms . Several studies described thymol and carvacrol inhibited growth of preformed biofilm and interfered with biofilm formation during planktonic growth [45,46]. Nostro et al.  reported carvacrol and thymol attenuated biofilm formation of S. aureus and Staphylococcus epidermidis strains on polystyrene microtitre plates and they suggested these oils repressed expansion of preformed biofilm and obstructed with the biofilm development during planktonic phase. El Abed et al.  also described anti-adherence and antibiofilm effects of terpenes and pointed out the excellent effectiveness of eugenol, carvone, and carveol, which could characterize candidates in the management of P. aeruginosa biofilm. Thymol can also prevent the first stages of biofilm formation and interfering with the formation of mature biofilms as a result of the inhabitation of metabolic activity for biofilms. All of these events may lead to major membrane and blockage the production of viable filamentous forms during the early steps of biofilm formation. As biofilms are multifactorial event, the several mechanisms of thymol (terpenes) perhaps effect on diverse stages in their development .
Antifungal effect by thymol and carvacrol
Direct antifungal agents resistance is still a chief unease when antifungal treatment failure is considered . There are limits antifungal drugs available for treatment, drug-resistant strains are also evidence of biofilm infections and side-effects of prescription drugs will have problems in the prevention and treatment of fungal infections . Several studies described antifungal effects of thymol and carvacrol against fungal pathogens. Antifungal effect of thymol and carvacrol investigated against Penicillium digitatum and Penicillum italicum. Both essential oils were effective in inhibiting fungal growth; thymol was more effective than carvacrol .
Guo et al.  indicated antifungal activity of thymol against clinical isolates of fluconazole susceptible and nonsusceptible Candida albicans and high percentage of synergism effects of thymol in combination with amphotericin B.
Thymol and carvacrol because of the restrain of ergosterol biosynthesis and the disturbance of membrane totality shows potent fungicidal efficacy against Candida isolates . Effective fungicidal properties of carvacrol and thymol against different plant pathogens were also formerly reported by Kordali et al. .
Development of herbicides helps reduce factors such as pollution and environmental degradation; in this regard, natural herbicides can be effective. Essential oils and monoterpenes compounds showed antifungal activity in the treatment of mucormycosi [54,55].
Thymol is lipophilic compound, that alone or with carvacrol, can change the cell membrane fluidity and permeability . In addition to this, the compound can changes the cell membrane in fungi such as C. albicans by the affect the function of the cell membrane enzymes that catalyzes the synthesis of the cell wall polysaccharide compounds such as β -glucan and inhibit the growth of cells [57,58]. The results of electron microscopy showed that thymol and carvacrol change the morphogenesis of the envelope of C. albicans .
Carvacrol was also effective in reducing the growth of Botrytis cinerea in berry and grapes; in grapes, 97% inhibition was related to the higher doses of carvacrol [59,60]. In addition to this, carvacrol was effective in reducing the spore germinates and mycelium growth of B. cinerea inoculated in grapes .
Other researchers showed the effect of monoterpenoidson the conidial germination and mycelial growth of B. cinerea . Also, Tsao and Zhou  reported that 0.25 mg/ml of thymol had an inhibitory effect on the increase of mycelium of Monilinia fructicola, also on the solid media, was 100% inhibited conidial germination of the bacteria.
Essential oils affect the various active molecules in the cell for different purposes, that, main purpose is the cytoplasmic membrane . Disruption of the permeability of the cell membrane leads to the loss of cell function such as the electron transport chain, also affected the eukaryotic cells . Toxicity to eukaryotic cells is responsible for undesirable side-effects for a host, such as inflammation, corrosion, cell sensitivity, acute toxicity to organs, and limits the use of essential oils as medicinal use . It is difficult to detect the toxicity of essential oils because the toxicity varies based on the compounds and depends on various factors . A study showed that thymol and carvacrol had the most toxic in concentrations of 36–49 mg/l, which are less toxic than some combination of essential oils . There is less risk of accumulation of body tissues. Therefore, it is suggested possible medical use thymol and carvacrol, but more research must be done on this issue.
Several studies have shown antibacterial and antifungal property of the thymol and carvacrol. Thymol and carvacrol can be applied as an alternative antimicrobial agent against antibiotic-resistant pathogenic bacteria and C. albicans. It is necessary for further precise detection of thymol and carvacrol safety to determine the optimal dose of these substances for human cells. Results of various studies proposed replace of traditional medicines instead of synthetic drugs, which has more side-effects. In this review, reported information about the effects of antibacterial, antifungal, and antibiofilm thymol and carvacrol that provides a better view about the thymol and carvacrol. More studies using bacterial strains isolated from patients treated with these compounds needs to be done.
This study was supported by Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran.
Conflicts of interest
The authors declare no conflicts of interest.
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