Not all microbes cause disease, in fact most of them don’t, but those that do are called “pathogenic”, meaning disease-causing. Pathogenic micro-organisms (PMs), in the form of bacteria and fungi, have plagued modern humans for 200,000 years. Bacteria, nearly as old as the Earth itself, were a dominant life form about 3 billion years ago, while fungi appeared on land about 560 million years ago. Over the millennia they’ve established invasive strategies to co-exist with other life forms, such as plants, mammals, birds, insects, and our ancestors, all of which have developed defensive strategies, in order to avoid disease.

Chemical defence systems
One defence against PMs is simply a mobile lifestyle, which decreases the number of interactions with them. Because plants are immobile, they have generated incredibly strong antibacterial and antifungal chemical defence systems, which often include essential oils (Helander, I. et al., 1998). With the evolution of defence mechanisms, even mobile life forms have either developed similar chemical defence systems, or have ‘borrowed’ plant defences to strengthen their strategies.

Chemical defence systems consist of chemical groups such as phenols, monoterpenes, sesquiterpenes, acids, alcohols, and aldehydes. Often, it is the synergy of individual chemical components that give bioactive life to a defensive-chemical blend. These blends can take many forms, including the essential oils that we use today (Nazzaro, F. et al., 2013). For example, mandarin (Citrus reticulata) essential oil alters the extracellular conductivity of bacteria resulting in growth inhibition (Tao, N. et al., 2014).

Citrus oils
Essential oils from citrus fruits are among the most effective in fighting bacteria. The essential oils of lemon (C. x limon) and bitter orange (C. aurantium) are used to defend some of the world’s major crops (e.g. cotton, cucumber, carrot, and tomato) against PMs (El-Zemity, S. et al., 2008; Chudasama, K. & Thaker, V., 2014). Harvested foods also benefit from citrus oils such as sweet orange (C. sinensis) and grapefruit (C. paradisi), of which limonene is the main component. These inhibit the growth of bacteria that cause food spoilage (Chaibi, A. et al., 1997). The dreadful Escherichia coli bacterium can be combated by sweet orange, lemon, and bergamot (C. bergamia) essential oils and their vapors – bergamot being the most effective oil, and linalool the most effective constituent (Fisher, K. & Phillips, C.A., 2006). Not only are these oils used as food additives to fight bacteria, but they also have the benefit of having a pleasing aroma and taste (Chang, S. et al., 2001).

Rose and citrus essential oils have also attracted attention as natural fungicides (Chutia, M. et al., 2009; Jurgens, A., 2010; Stevic, T. et al., 2014). Six different types of fungi are completely inhibited by the combination of clove (Syzygium aromaticum) and cinnamon (Cinnamomum spp) oils, as well as five essential oil components found in citrus and rose oils: citral, eugenol, geraniol, limonene, and linalool (Kishore, G. & Pande, S., 2007). Although citrus oils have strong antifungal properties for other plants, citrus fruit itself also needs to be protected from fungal growth. The essential oil and hydrosol of headed thyme (Thymus capitatus L.) keeps sweet oranges fungus-free (Tabti, L. et al., 2014).

Self-anointing
Despite the fact that mammals can defend themselves against PMs by mobility, some also defend themselves chemically by a process of self-anointing. In self-anointing, an animal spreads a smelly substance over its body that can act both as a repellent and as an antibacterial or antifungal agent. For example, wolves (Zimen, E., 1981), capuchins (Leca, J. et al., 2007) (Fig. 1), and coatis use the peels, pulp, and juice of limes and lemons to anoint their bodies to deter and inhibit infective organisms (Weldon, P. et al., 2011).

Figure 1. Left: a tufted capuchin monkey (Cebus apella) rubs a lemon slice against its back; Center: a boat-tailed grackle (Quiscalus major) rubs a lemon slice against its wing and trunk; Right: a whitenosed coati (Nasua narica) rubs a lemon slice against its tail

Birds also self-anoint. Grackles use lime to deter ectoparasites and it is believed that the lime also functions as an antibacterial agent. Blue tits don’t self-anoint, but they protect their nests against PMs with aromatic plants such as Helichrysum italicum, Melissa officinalis, Achillea ligustica, and Lavandula stoechas. The bioactive components of these plants include limonene, linalool, and terpinen-4-ol (Petit, C. el., 2002). Citrus oils share some of the same antimicrobial constituents as are found in these plants. Were these birds to live among citrus trees, perhaps they would implement these too in their defence repertoire. Not only do essential oils attack “bad” bacteria, but as with the tangerine-scented bird, the good bacteria are thought to break down the secretion that results in a citrusy tangerine-like aroma that attracts potential mates and deters ectoparasites (Nature News, 2003).

Just as there are “good” and “bad” bacteria, there are also “good” and “bad” fungi. Many creatures depend on fungus for survival; therefore its health, sustainability, and protection are vital. Just as fungi are an integral part of our ecosystem, so are the ants and many other life forms that use it as sustenance. There is a symbiotic relationship between fungus and leaf cutting ants, whereby the ants rely on healthy fungus colonies as a food source, and the fungus is protected against parasitic organisms by the citral- and geraniol- rich ‘essential oil’ secreted by the ants mandibular and metapleural glands. As they chew up leaves for their fungal colonies to grow on, the leave mulch is injected with ant “essential oil” (Mendonca, A. et al., 2009).

We, like plants, mammals, birds, and insects, not only need protection against PMs (see recipe below), but our health and overall wellbeing thrive on the beneficial properties of essential oils. We not only have limitless mobility, but we also have a deepening understanding of, and respect for, those plants that have given us the gift of chemical defence.

References
Chaibi, A., Ababouch, L.H., Belasri, S., Boucetta, S., and Busta, F.F. 1997. Inhibition of germination and vegetative growth of Bacillus cereus T and Clostridium botulinum 62A spores by essential oils. Food Microbiology 14, pp. 161-174.

Chang, S-T., Chen, P-F., and Chang, S-W. 2001. Antibacterial activity of leaf essential oils and their constituents from Cinnamomum osmophloeum. Journal of Ethnopharmacology 77, pp. 123-127.

Chudasama, K. and Thaker, V. 2014. Biological control of phytopathogenic bacteria Pantoea agglomeransand Erwinia chrysanthemi using 100 essential oils. Archives of Phytopathology and Plant Protection 47 (18), pp. 2221-2232.

Chutia, M., Bhuyan, P., Pathak, M.G., Sama, T.C., and Boruah, P. 2009. Antifungal activity and chemical composition of Citrus reticulata Blanco essential oil against phytopathogens from North East India. LWT-Food Science and Technology 42, pp. 777-780.

El-Zemity, S., Radwan, M., El-Monam Mohamed, and Sherby, S. 2008. Antibacterial screening of some essential oils, monoterpenes and novel N-methyl carbamates based on monoterpenoids against Agrobacterium tumefaciens and Erwinia carotovora. Archives of Phytopathology and Plant Protection 41(6), pp. 451-461.

Fisher, K. and Phillips, C.A. 2006. The effect of lemon, orange, and bergamot essential oils and their components on the survival of Campylobacter jejuni, Escherichia coli 0157, Listeria monocytogenes, Bacillus cereus and Staphylococcus aureus in vitro and in food systems. Journal of Applied Microbiology 101, pp. 1232-1240.

Helander, I.M., Alakomi, H., Latva-Kala, K., Mattila-Sandholm, T., Pol, I., Smid, E.J., Gorris, L., von Wright, A. 1998. Characterization of the action of selected essential oil components on gram-negative bacteria. Journal of Agricultural and Food Chemistry 46 (9), pp. 3590-3595.

Jurgens, A. 2010. Chemical diversity and biological functions of plant volatiles. South African Journal of Botany 76, pp. 607-611.

Kishore, G., and Pande, S. 2007. Evaluation of essential oils and their components for braod-spectrum antifungal activity and control of late leaf spot and crown rot diseases in peanut. The American Phytopathological Society: Plant Disease 91 (4), pp. 375-379.

Leca, J., Gunst, N. and Petit, O. 2007. Social aspects of fur-rubbing in Cebus capucinus and C. apella. International Journal of Primatology 28, pp. 807-817.

Mendonca, A., Silva, C., Mesquita, F., Campos, R., Nascimento, R., Ximenes, E., and Sant’Ana, A. 2009. Antimicrobial activities of components of the glandular secretions of leaf cutting ants of the genus Atta. Antonie van Leeuwenhoek 95, pp. 295-303.

Nature News. 2003. Citrus smell attracts seabirds.

Nazzaro, F., Fratianni, F., Martino, L., Coppola, R., and Feo, V. 2013. Effect of essential oils on pathogenic bacteria. Pharmaceuticals 6, pp. 1451-1474.

Petit, C., McKey, M., Perret, P., Blondel, J., Lambrechts, M. 2002. Blue tits use selected plants and olfaction to maintain an aromatic environment for nestlings. Ecology Letters 5, pp. 585-589.

Stevic, T., Beric, T., Savikin, K., Sokovic, M., Godevac, D., Dimkic, I., and Stankovic, S. 2014. Antifungal activity of selected essential oils against fungi isolated from medicinal plant. Industrial Crop and Products 55, pp. 116-122.

Tabti, L., Dib, M., Djabou, N., Benyelles, N., Paolini, J., Costa, J., Muselli, A. 2014. Control of fungal pathogens of Citrus sinensis L. by essential oil and hydrosol of Thymus capitatus L. Journal of Applied Botany and Food Quality 87, pp. 279-285.

Tao, N., and Zhou, L. 2014. Anti-fungal activity of Citrus reticulata Blanco essential oil against Penicillium italicum and Penicillium digitatum. Food Chemistry 153, pp. 265-271.

Weldon, P., Carroll, J. Kramer, M., Bedoukian, R., Coleman, R., and Bernier, U. 2011. Anointing chemicals and hematophagous arthropods: Responses by ticks and mosquitoes to citrus (Rutaceae) peel exudates and monoterpene components. Journal of Chemical Ecology 37, pp. 348-359.

Zimen, E. 1981. The wolf: A species in danger. Delacorte, New York.