Origanum is a genus of herbaceous perennials and subshrubs in the family Lamiaceae, native to Europe, North Africa, and much of temperate Asia, where they are found in open or mountainous habitats. A few species also naturalized in scattered locations in North America and other regions. The plants have strongly aromatic leaves and abundant tubular flowers with long-lasting coloured bracts. The genus includes the important group of culinary herbs: marjoram (Origanum majorana) and oregano (Origanum vulgare). Origanum species are used as food plants by the larvae of some Lepidoptera species, including Coleophora albitarsella.
The Origanum (Lamiaceae) genus is an annual, perennial and shrubby herb that is predominantly distributed around the Mediterranean, Euro-Siberian, and Iran-Siberian regions. Its use in traditional medicine has given rise to numerous studies that confirmed the benefits of oregano for human health, and its oil has been used to treat respiratory and gastrointestinal disorders, as well as an oral antiseptic and in dermatological applications. Origanum has around 39 species, of which Origanum vulgare L. is the most studied. Its essential oil has been reported to have several biological properties, such as antioxidant, antimicrobial, and antimutagenic.
Origanum vulgare essential oil is composed of different compounds. Most of them are thymol and carvacrol, but other compounds include ρ-cymene, thymoquinone, and γ-terpinene. It has been reported that oregano essential oils have a powerful antimicrobial action against bacteria (Gram-positive and Gram-negative), yeast and fungi. Oregano (Origanum vulgare subsp. vulgare) essential oil showed activity against Sarcina lutea, S. aureus, C. albicans, E. faecalis, and Bacillus cereus, resulting in inhibition halos of 34.67, 26.67, 24.67, 22.33, and 20.33 mm, respectively.35 Lv et al.37 reported that oregano essential oil has high activity against S. aureus (27.4±0.5 mm), B. subtilis (27.4±0.7 mm), E. coli (18.2±0.8 mm), and Saccharomyces cerevisiae (27.2±0.6 mm). In the same study, the MIC was determined for all tested microorganisms, they found that MIC was the lowest with 0.625 µL/mL, in all bacterial strains. Hammer et al.29 reported similar results, MIC values of oregano oil against E. coli and S. aureus were from 0.5 to 1.2 µL/mL. Actually, oregano has been used against pathogenic bacterial strains such as E. coli O157:H7, and when it was in direct contact with Salmonella Typhimurium DT104, this microorganism was inactivated.
Oregano essential oil has been tested for use as a cleaning agent in the treatment of antibiotic-resistant bacteria on organic leafy greens; an investigation demonstrated that with a treatment of 0.5% oregano oil, the greatest microorganism population reductions (up to 4.9-log) was seen on all leafy greens, and even biological activity increased over time in storage. In this context, it was investigated the disinfection efficacy of oregano oil on Salmonella Typhimurium inoculated into iceberg lettuce. Washing lettuce leaves with oregano oil led to a significant reduction in numbers of Salmonella Typhimurium as compared with chlorinated water. The best result occurred at a 75-ppm concentration, with a reduction of 1–92 log CFU/g. The authors stated that oregano oil could be used as a natural alternative to chlorine without affecting sensory properties.
Some studies have reported the effect of oregano oil in combination with other essential oils like marjoram, where the reduction in the maximum specific microbial growth rate achieved was approximately threefold higher than that with the oregano oil alone.Investigations have further examined the effect of oregano essential oil on lag phase duration; when E. coli was exposed to oregano in combination with basil, the results indicated that the time of the lag phase increased by 7.44 h with respect to the increase with oregano alone.