The chemical anthropology of antimicrobial plants

Townsend Letter for Doctors and Patients, July, 2004 by Tim Batchelder

Introduction

Traditional people have been using powerfully antimicrobial plants for thousands of years as part of their diet and pharmaceutical arsenal. Recently, people in industrialized nations have begun to express interest in these natural products. However, Hospital-based medicine has been slow to embrace medicinal plants and extracts as a source of drugs. For example, an article by M. Cowan in Clinical Microbiology Reviews (October 1999) notes that while 25 to 50% of current pharmaceuticals are derived from plants, none are used as antimicrobials. Cowan notes that plants are rich in a wide variety of secondary metabolites, such as tannins, terpenoids, alkaloids, and flavonoids, which have been found to have antimicrobial properties. Further, while it is estimated that there are 250,000 to 500,000 species of plants on Earth, a small percentage (1 to 10%) of these are used as foods by both humans and other animal species, leaving a huge potential for medicinal plant product development. In this article, I'll explore the chemical anthropology of anti-microbial plants in greater detail drawing on Cowan's landmark review.

[ILLUSTRATION OMITTED]

Growing Interest

Cowan notes that since the advent of antibiotics in the 1950s, the use of plant derivatives as antimicrobials has been minimal. It is reported that, on average, two or three antibiotics derived from microorganisms are launched each year. Yet he notes that after downturn in recent decades, the pace is again quickening as scientists realize that the life span of conventional antibiotics (products of microorganisms or their synthesized derivatives) is limited and as new, particularly viral, diseases remain intractable to this type of drug. Human immunodeficiency virus (HIV) triggered investigation into the plant derivatives which may be effective, especially for use in underdeveloped nations with little access to expensive Western medicines. In one study glycyrrhizin, found in Glycyrrhiza plants (the source of licorice), extended the life of the retrovirus-infected mice from 14 to 17 weeks. A crude extract of the cactus Opuntia streptacantha had marked antiviral effects in vitro, and toxicity studies performed in mice, horses, and humans found the extract to be safe. Another reason for the renewed interest in plant antimicrobials in the past 20 years has been the rapid rate of species extinction. Worldwide spending on finding new anti-infective agents (including vaccines) in 1999 was expected to increase 60% from the spending levels in 1993. Cowan notes that in 1996, sales of botanical medicines increased 37% over 1995. It is speculated that the American public may be reacting to overprescription of sometimes toxic drugs, just as their predecessors of the 19th century reacted to the overuse of bleeding, purging, and calomel.

History and Pre-History

Cowan gives a brief history of antimicrobial plant use. Neanderthals living 60,000 years ago in present-day Iraq used plants such as hollyhock. Hippocrates (in the late fifth century BC) mentioned 300 to 400 medicinal plants. In the first century AD, Dioscorides wrote De Materia Medica, a medicinal plant catalog which became the prototype for modern pharmacopoeias. The Bible offers descriptions of approximately 30 healing plants such as frankincense and myrrh which have antiseptic properties and were employed as mouthwashes. European culture lost much of its antimicrobial plant knowledge with the fall of its civilizations but then regained interest with the Renaissance. During the Dark Ages the Arab world excavated their own medicinal plant literature and Asian cultures also compiled their own pharmacopoeia. In more modern times, many traditional folk songs and tales emphasize the importance of consuming anti-microbial plants on a seasonal basis. For example "Eat leeks in March and wild garlic in May, and all the year after the physicians may play" is a traditional Welsh rhyme. Moerman reported that while 1,625 species of plants have been used by various Native American groups as food, 2,564 are used as drugs. After the Native American cultures an "alternative" movement among Americans of European origin, began in the 19th century, in which they refused "toxic" mainstream medicinal practices of the day. Oliver Wendell Holmes noted that medical treatments in the 1800s could be dangerous and ineffective such as the use of mercury baths in London "barber shops" to treat syphilis and dangerous hallucinogens as a tuberculosis "cure" and in 1861 advocated abandoning the whole materia medica. In 1887, alternative practitioners compiled their own catalogs, such as The Homeopathic Pharmacopoeia of the United States.

Chemical Groupings

Cowan notes that plants have an almost limitless ability to synthesize aromatic substances, most of which are phenols or their oxygen-substituted derivatives. Most are secondary metabolites, of which at least 12,000 have been isolated, less than 10% of the total. These substances serve as plant defense mechanisms against predation by microorganisms, insects, and herbivores. Some, such as terpenoids, give plants their odors; others (quinones and tannins) are responsible for plant pigment; others give plants flavor (e.g., the terpenoid capsaicin from chili peppers).