گیاهان دارویی و فعالیت ضد میکروبی Medicinal plants and antimicrobial activity

۱٫ Introduction Long before mankind discovered the existence of microbes, the idea that certain plants had healing potential, indeed, that they contained what we would currently characterize as antimicrobial principles, was well accepted. Since antiquity, man has used plants to treat common infectious diseases and some of these traditional medicines are still included as part of the habitual treatment of various maladies. For example, the use of bearberry (Arctostaphylos uva-ursi) and cranberry juice (Vaccinium macrocarpon) to treat urinary tract infections is reported in different manuals of phytotherapy, while species such as lemon balm (Melissa officinalis), garlic (Allium sativum) and tee tree (Melaleuca alternifolia) are described as broad-spectrum antimicrobial agents (Heinrich et al., 2004). That being said, it has generally been the essential oils of these plants rather than their extracts that have had the greatest use in the treatment of infectious pathologies in the respiratory system, urinary tract, gastrointestinal and biliary systems, as well as on the skin. In the case of Melaleuca alternifolia, for example, the use of the essential oil (tee tree oil) is a common therapeutic tool to treat acne and other infectious troubles of the skin (Vanaclocha and Canigueral, 2003 ˜ ). In the present paper, we analyze the past, present and future of medicinal plants, both as potential antimicrobial crude drugs as well as a source for natural compounds that act as new anti-infection agents. ۲٫ Past In the past few decades, the search for new anti-infection agents has occupied many research groups in the field of ethnopharmacology. Recio et al. (1989a) reviewed the most relevant articles on this subject published between 1978 and 1988, compiling a list of 75 species in which the authors had established the activity of the extract along with both the spectrum of and the principles responsible for this activity. In general, the review showed that phenolics are the predominant active chemical in these plants, with Gram positive bacteria being the most sensible germs. The review also revealed, however, the major problem with this type of research, namely the lack of uniformity in the criteria selected to study the activity. This has in the past lead to relevant contradictions between the results obtained by different groups and even for the same authors studying the same sample with different methods (Pellecuer et al., 1976). To try to solve this important problem, R´ıos et al. (1988) published a review of the experimental methods used for studying the activity of both plant extracts and essential oils to date. They proposed the use of diffusion methods for studying polar compounds of small or medium molecular size and determining the antimicrobial spectrum because this method allows researchers to test different compounds against one microorganism. The solid dilution method was recommended for studying polar and non-polar substances as well as all types of complex extracts. This method is especially good for determining the relative potency of extracts or essential oils and for establishing their antimicrobial spectrum as it facilitates the use of different strains against the extract on the same plate. Finally, the liquid dilution method is the best way to establish the real potency of a pure compound, but solubility is an obvious requisite. The protocols proposed by R´ıos et al. were widely accepted by many research groups as can be seen by the numerous times the paper has been cited. The proposed methods have been used principally in the study of plant extracts of medium or no polarity. The article has been most often cited in articles published in the Journal of Ethnopharmacology (23% of all the citations of the article), followed by those appearing in Pharmaceutical Biology (12%), but all told it has been cited in 42 journals, some of them extremely specific. To examine the problem of a lack of unified criteria more in depth, we can look particularly at the study of the antimicrobial activity of essential oils. Janssen et al. (1987) reviewed the characteristics of these kinds of complex mixtures as well as the techniques used for studying them and concluded that the results are difficult to compare as the test methods differed so widely. They thus proposed that in future the strain number of the tested microorganism, the composition of the essential oil and the conditions under which it was obtained be included as an integral part of the report. Recently, Kalemba and Kunicka (2003) reviewed the classical methods commonly used for the evaluation of the antibacterial and antifungal activities of essential oils, including the agar diffusion method (paper disc and well), the dilution method (agar and liquid broth) and the turbidimetric and impedimetric monitoring of microorganism growth in the presence of tested essential oils to draw conclusions about the factors that influence the in vitro antimicrobial activity of essential oils and their mechanisms of action. Moreover, they include an overview of the susceptibility of human and foodborne bacteria and fungi towards different essential oils and their constituents. The most relevant ones, which include the essential oils of thyme, origanum, mint, cinnamon, salvia and clove, have antimicrobial properties.