The use of and search for drugs and dietary supplements derived from plants have accelerated in recent years. Ethnopharmacologists, botanists, microbiologists, and natural-products chemists are combing the Earth for phytochemicals and “leads” which could be developed for treatment of infectious diseases. While 25 to 50% of current pharmaceuticals are derived from plants, none are used as antimicrobials. Traditional healers have long used plants to prevent or cure infectious conditions; Western medicine is trying to duplicate their successes. Plants are rich in a wide variety of secondary metabolites, such as tannins, terpenoids, alkaloids, and flavonoids, which have been found in vitro to have antimicrobial properties. This review attempts to summarize the current status of botanical screening efforts, as well as in vivo studies of their effectiveness and toxicity. The structure and antimicrobial properties of phytochemicals are also addressed. Since many of these compounds are currently available as unregulated botanical preparations and their use by the public is increasing rapidly, clinicians need to consider the consequences of patients self-medicating with these preparations.

Plant Products as Antimicrobial Agents

Biological and Phytochemical Screening of Plants

Populations of plants are subject to attack by pathogens and herbivores at all stages of their life cycles. Pest pressure may be devastating, as illustrated by the outbreaks of insects apd diseases in cultivated and native plants (121, 173, 178, 198, 329) and in the success of weed control by biotic instruments (6, 141, 338, 340). Epidemics or episodes of intensive herbivory may lead directly to the local extinction of a plant species, or may weaken plants and make them vulnerable to subsequent pressures. Loss of photosynthetic capacity due to disease or partial defoliation lowers resistance to abiotic stresses or to infection by other pests, and reduces competitive ability (198, 240, 270). Defense against pathogens and herbivores may be secured by many different mechanisms; it may depend on the texture and composition of the plant surface (156, 182, 196, 242), the presence of anatomical structures such as thorns or resin ducts (123, 199, 229), the absence of nutrients required by the pest (140), the presence of hormonelike substances that affect the development of insects (335), unsuitable pH or osmotic pressure (18, 199, 229), or the accumulation of secondary products (47, 92, 169, 298). These products are diverse, ranging from alkaloids to terpenes; phenolics; and steroidal, cyanogenic, and mustard oil glycosides. Some (e.g. phenolics) are ubiquitous in seed plants, others (e.g. alkaloids) occur in several thousand species and others (e.g. cyanogenic glycosides) occur in only a few hundred species (61). Secondary products may be sequestered within cells or organelles therein and released only when the cells are broken. They also may be stored in and secreted by epidermal glands which are sessile or on trichomes (19, 69, 156, 182, 187, 269). Typically, glandular exudates contain only one class of compound, but this class varies among species. The glandular exudates usually are terpens, alkaloids, res-

The Chemical Defenses of Plants to Pathogens and Herbivores

Plant antimicrobials are not used as systemic antibiotics at present. The main reason for this is their low level of activity, especially against gram-negative bacteria. The reported MIC is often in the range of 100 to 1,000 μg/ml, orders of magnitude higher than those of common broad-spectrum antibiotics from bacteria or fungi. Major plant pathogens belong to the gram-negative bacteria, which makes the low level of activity of plant antimicrobials against this group of microorganisms puzzling. Gram-negative bacteria have an effective permeability barrier, comprised of the outer membrane, which restricts the penetration of amphipathic compounds, and multidrug resistance pumps (MDRs), which extrude toxins across this barrier. It is possible that the apparent ineffectiveness of plant antimicrobials is largely due to the permeability barrier. We tested this hypothesis in the present study by applying a combination of MDR mutants and MDR inhibitors. A panel of plant antimicrobials was tested by using a set of bacteria representing the main groups of plant pathogens. The human pathogens Pseudomonas aeruginosa, Escherichia coli, and Salmonella enterica serovar Typhimurium were also tested. The results show that the activities of the majority of plant antimicrobials were considerably greater against the gram-positive bacteria Staphylococcus aureus and Bacillus megaterium and that disabling of the MDRs in gram-negative species leads to a striking increase in antimicrobial activity. Thus, the activity of rhein, the principal antimicrobial from rhubarb, was potentiated 100- to 2,000-fold (depending on the bacterial species) by disabling the MDRs. Comparable potentiation of activity was observed with plumbagin, resveratrol, gossypol, coumestrol, and berberine. Direct measurement of the uptake of berberine, a model plant antimicrobial, confirmed that disabling of the MDRs strongly increases the level of penetration of berberine into the cells of gram-negative bacteria. These results suggest that plants might have developed means of delivering their antimicrobials into bacterial cells. These findings also suggest that plant antimicrobials might be developed into effective, broad-spectrum antibiotics in combination with inhibitors of MDRs.

Multidrug Pump Inhibitors Uncover Remarkable Activity of Plant Antimicrobials

Taito O. Soine

Papers

Naturally Occurring Coumarins and Related Physiological Activities

Coumarins I: Isolation, Purification, and Structure Determination of Pteryxin and Suksdorfin

Coumarins X: Spectral Studies on Some Linear Furanocoumarins

Coumarins II. structures of columbianadin and columbianin

The Structure of Argemonine; Identification as (-)N-Methylpavine