According to World Health Organization (WHO) more than 80% of the world's population relies on traditional medicine for their primary healthcare needs. Use of herbal medicines in Asia represents a long history of human interactions with the environment. Plants used for traditional medicine contain a wide range of substances that can be used to treat chronic as well as infectious diseases. A vast knowledge of how to use the plants against different illnesses may be expected to have accumulated in areas where the use of plants is still of great importance 1. The medicinal value of plants lies in some chemical substances that produce a definite physiological action on the human body. The most important of these bioactive compounds of plants are alkaloids, flavanoids, tannins and phenolic compounds 2.

Rural communities, in particular paliyar tribes, depend on plant resources mainly for herbal medicines, food, forage, construction of dwellings, making household implements, sleeping mats, and for fire and shade. The use of medicinal plants as traditional medicines is well known in rural areas of many developing countries 34. Traditional healers claim that their medicine is cheaper and more effective than modern medicine. In developing countries, low-income people such as farmers, people of small isolate villages and native communities use folk medicine for the treatment of common infections 5.

We chose eighteen plant species used in folk medicine to determine their antimicrobial activity (Table 1). In general, these plants are used in folk medicine in the treatment of skin diseases, venereal diseases, respiratory problems and nervous disorders. Properties of the collected plants are also provided in same table. Evidently, there are not many scientific studies that confirm the antimicrobial properties for most of the plants collected for this study. The phytochemical research based on ethnopharmacological informations is generally considered an effective approach in the discovery of new anti-infective agents from higher plants 6.

There are a few reports on the use of plants in traditional healing by either tribal people or indigenous communities of Tamil Nadu 7891011. The development of drug resistance in human pathogens against commonly used antibiotics has necessitated a search for new antimicrobial substances from other sources including plants 12. Screening of medicinal plants for antimicrobial activities and phytochemicals is important for finding potential new compounds for therapeutic use. This paper reports the results of a survey that was done based on folk uses by traditional practitioners in Palni hills of Tamil Nadu along with bioassay test for antimicrobial activity.

Table 1 provides the botanical name, family, local name, plant parts used together with their traditional therapeutic uses and properties for the 18 ethnomedicinal plants collected from Palni hills of Tamil Nadu. Out of the 18 plants tested for antimicrobial activity, 10 plant species showed antimicrobial activity by inhibiting one or more microorganisms. The results of the antimicrobial screening of the crude extracts of all species of plants are shown in Table 2. Among the plants screened, Acalypha fruticosa, Peltophorum pterocarpum, Toddalia asiatica,Cassia auriculata, Syzygium cumini and Syzygium lineare showed promising activity against tested microorganisms. The tested plant extracts were most active against gram-positive microorganisms than gram-negative microorganisms. This is in agreement with previous reports by the several workers 141516171819.

Methanol extracts exhibited a higher degree of antimicrobial activity as compared with hexane extracts. Methanol extracts of Albizia procera,Cassia auriculata (leaves and flowers), Peltophorum pterocarpum, Punica granatum and Syzygium cumini showed activity. Punica granatum possesed 25% of tannin 20 and the antibacterial activity may be indicative of the presence of some metabolic toxins or broad-spectrum antibiotic compounds. Also ethanolic extract of Punica granatum was most active against E. coli. Prasanth et al 21 reported that, different extracts of Punica granatum fruit showed some antibacterial activity against P.vulgaris and B.subtilis. Rajakaruna et al 22 reported that Syzygium cumini showed good activity against Staphylococcus aureus and Bacillus subtilis.

Both hexane and methanol extracts of Syzygium lineare and Toddalia asiatica showed antimicrobial activity. The essential oils from the leaves of Toddalia asiatica were most active against E. coli, K. pneumoniae, P. aeruginosa and S. aureus 23. Peltophorum pterocarpum and Syzigium lineare had the highest inhibitory activity against both gram positive and gram negative bacteria. On the other hand, Cassia alata showed only slight activity against bacteria such as S. aureus and B. subtilis. In the previous findings leaves, flowers, root and stem barks of Cassia alata showed a range of activity against several bacteria and protozoa 24. In this study methanol extract of leaves of Cassia alata showed antibacterial activity against Bacillus subtilis and Staphylococcus aureus. Somchit et al 25 also tested the whole plant parts of Cassia alata and showed activity in the leaves against Staphylococcus aureus.

Syzygium lineare, Punica granatum, Syzygium cumini and Toddalia asiatica produced the largest zones of inhibition against Bacillus subtilis, Staphylococcus aureus and Staphylococcus epidermidis. Voravuthikunchai et al 20 reported good antibacterial activity in P. pterocarpum and P. granatum against Escherichia coli using aqueous and methanol extracts. Methanol extracts of Peltophorum pterocarpum and Punica granatum showed activity against Candida albicans. In general, among the tested microbial strains, bacteria were found to be more sensitive to many of the test agents than fungi.

The most sensitive bacterium was Bacillus subtilis, which was inhibited by methanol or hexane of 10 plants. On the other hand, no inhibition was observed in the Eruvinia sps. Some organisms exhibited only slight susceptibility. E. coli was inhibited by methanol extract of flowers of Cassia auriculata and hexane extract of Punica granatum. Proteus vulgaris was inhibited by methanol extract of Peltophorum pterocarpum and Syzigium lineare. Klebsiella pneumonia was inhibited by hexane extract of Olax scandens, methanol extracts of Peltophorum pterocarpum and Syzigium cumini.

The processing of the plants performed in this study was not comparable to the traditional approach when the Paliyar tribe used water for extracts whereas we have used hexane and methanol for extraction. In this sense it is not an exact replication of the traditional knowledge. All the same, given that methanol extracts were more effective then hexane extract, it is likely that water extracts were will be effective as well and possibly mort so.

The antibacterial activity of Syzigium lineare, Olax scandens, Albizia procera and Acalypha fruticosa are reported for the first time. No previous report on the antibacterial activity of these species could be found in the literature. Among the medicinal plants tested in this work, Peltophorum pterocarpum and Punica granatum showed the most promising antimicrobial properties indicating the potential for discovery of antibacterial principles. Further phytochemical studies are required to determine the types of compounds responsible for the antibacterial effects of these species. The results also indicate that scientific studies carried out on medicinal plants having traditional claims of effectiveness might warrant fruitful results. Several plants used by Paliyar tribe exhibit some degree of antibacterial activity towards gram-positive bacteria such as, Bacillus subtilis, Staphylococcus aureus and Staphylococcus epidermidis. These plants could serve as useful sources for new antimicrobial agents.

The author(s) declare that they have no competing interests.

VD has carried out the experimental part such as extraction, inoculum preparation and antimicrobial evaluation. MA collected the ethnomedicinal plants from the Palni hills of Tamil Nadu with the help of tribal practitioners. SI supervised the work, evaluated the results and corrected the manuscript for publication. All authors read and approved the final manuscript.