Antifungal activity and mechanism of thymol against Fusarium oxysporum, a pathogen of potato dry rot, and its potential application

Thymol (2-isopropyl-5-methylphenol) is the main monoterpene phenol occurring in essential oils isolated from plants belonging to the Lamiaceae family (Thymus, Ocimum, Origanum, and Monarda genera), and other plants such as those belonging to the Verbenaceae, Scrophulariaceae, Ranunculaceae, and Apiaceae families. These essential oils are used in the food industry for their flavouring and preservative properties, in commercial mosquito repellent formulations for their natural repellent effect, in aromatherapy, and in traditional medicine for the treatment of headaches, coughs, and diarrhea. Many different activities of thymol such as antioxidant, anti-inflammatory, local anaesthetic, antinociceptive, cicatrizing, antiseptic, and especially antibacterial and antifungal properties have been shown. This review aims to critically evaluate the available literature regarding the antibacterial and antifungal effects of thymol.

The viable counts of Salmonella typhimurium on nutrient agar (NA) decreased upon the addition of either the essential oil of thyme or its constituent thymol, especially under anaerobic conditions. Antagonistic effects of thymol against Staphylococcus aureus were also greater under anaerobic conditions. In contrast to the phenolic constituents of the oil, thymol and carvacrol, the chemically related terpenes p-cymene and gamma-terpinene had no antagonistic effects against Salm. typhimurium. The addition of Desferal to NA counteracted the antibacterial effects of both thyme oil and thymol. No support was obtained, however, for a possible role of iron in the oxygen-related antibacterial action of the thyme oil and thymol or for the observed effect of Desferal. In the presence of thymol, the viable counts of Salm. typhimurium obtained on a minimal medium (MM) were lower than those obtained on NA. Addition of bovine serum albumin (BSA) neutralized the antibacterial action of thymol. It is suggested that the effects of BSA or Desferal are due to their ability to bind phenolic compounds through their amino and hydroxylamine groups, respectively, thus preventing complexation reactions between the oil phenolic constituents and bacterial membrane proteins. This hypothesis is supported by the marked decrease in the viable counts of Salm. typhimurium caused by either thyme oil or thymol when the pH of the medium was changed from 6.5 to 5.5 or the concentration of Tween 80 in the medium was reduced.

Thymol, chemically known as 2-isopropyl-5-methylphenol is a colorless crystalline monoterpene phenol. It is one of the most important dietary constituents in thyme species. For centuries, it has been used in traditional medicine and has been shown to possess various pharmacological properties including antioxidant, free radical scavenging, anti-inflammatory, analgesic, antispasmodic, antibacterial, antifungal, antiseptic and antitumor activities. The present article presents a detailed review of the scientific literature which reveals the pharmacological properties of thymol and its multiple therapeutic actions against various cardiovascular, neurological, rheumatological, gastrointestinal, metabolic and malignant diseases at both biochemical and molecular levels. The noteworthy effects of thymol are largely attributed to its anti-inflammatory (via inhibiting recruitment of cytokines and chemokines), antioxidant (via scavenging of free radicals, enhancing the endogenous enzymatic and non-enzymatic antioxidants and chelation of metal ions), antihyperlipidemic (via increasing the levels of high density lipoprotein cholesterol and decreasing the levels of low density lipoprotein cholesterol and low density lipoprotein cholesterol in the circulation and membrane stabilization) (via maintaining ionic homeostasis) effects. This review presents an overview of the current in vitro and in vivo data supporting thymol’s therapeutic activity and the challenges concerning its use for prevention and its therapeutic value as a dietary supplement or as a pharmacological agent or as an adjuvant along with current therapeutic agents for the treatment of various diseases. It is one of the potential candidates of natural origin that has shown promising therapeutic potential, pharmacological properties and molecular mechanisms as well as pharmacokinetic properties for the pharmaceutical development of thymol.