328 Activities and screening of Yemeni medicinal plant different types of active constituents like flavonoids, terpenoids, tannins, volatile oils, etc. (Table 3). Discussion The results of our screening assays confirmed the use of the investigated plants in Yemeni traditional medicine. It is the first report about antimicrobial, antioxidant and cytotoxic effects of A. semiflabellata, F. tinacissima, I. albicans, J. variegata, P. hadiensis, T. pungens and V. bottae. Whereas other plants like A. fruticosa, A. orientalis, D. viscosa, L. pubescens, L. citriodora and M. longifolia are partly well investigated. The existing knowledge about the other investigated plants is in many cases very limited. The antibacterial effect of A. fruticosa was investigated among some Indian medicinal plants (24). It was found that A. fruticosa was one of the most active plants tested. Other species of Acalypha like A. siamensis and A. wilkesiana also showed antimicrobial activity (25,26). A search on A. guatemalensis demonstrated antioxidant and antimicrobial activity for this plant (27). The studies on A. wilkesiana and A. hispida as well as A. communis demonstrated the isolation of gallic acid, corilagin, geraniin and triterpenoids of cycloartane-type as com- pounds responsible for the observed antimicrobial activity (28,29). Flavonoids like quercetin- and kaemp- ferol-derivatives were also identified. Thus, the estimated antimicrobial and antioxidant effects of the investigated A. fruticosa are in accordance with these data. Our phytochemical screening revealed the presence of terpe- noids and flavonoids in the methanolic extract of A. fruticosa, which could be responsible for these noteworthy activities. In previous work (10), we reported about the anticancer potential effect of A. fruticosa against five cancer cell-lines. The methanolic extract showed a moderate cytotoxic effect against different cancer cell-lines (IC50 > 50 mg ml 1) (10). The cytotoxicity of A. fruticosa against FL-cells with IC50 of 70 mg ml 1 was in accordance with that result. It was reported that the methanolic extract of D. viscosa has an antimicrobial effect (30,31). Others described the isolation of diterpenoid- and flavonoid- derivatives from D. viscosa (32–34). Our phytochemical screening indicated the presence of these types of compounds, which are mainly responsible for the remarkable antioxidant and antimicrobial effect of this plant. Whereas no reports about J. variegata were found, the extracts of other species of Jatropha namely J. elliptica and J. gossypifolia, exhibited antibacterial effect against gram-positive bacteria (35,36). The high antibacterial effect of the investigated L. citriodora against both antibiotic susceptible and resistant Gram-positive microorganisms is due to the high content of volatile oil. In earlier studies, it was found that the extracts and isolated volatile oil from L. citriodora and other Lippia species show a strong antimicrobial activity against Helicobacter pylori (37) and against different types of bacteria and fungi (38). Unlike this result, our investigated extract of L. citriodora exhibited activity only against Gram-positive and multi- resistant bacteria. No effect was observed against C. maltosa. In addition, it was demonstrated that the infusion of L. citriodora has a potent superoxide radical scavenging activity and a moderate scavenging activity of hydroxyl radical (39). The scavenging activity of DPPH radical in our screening was shown only at the highest concentration (1000 mg ml 1). This effect is due to the presence of several flavonoids and phenolic acids (39,40). For P. hadiensis, no reports were found. However several Plectranthus species were investigated for their antimicrobial activity against different types of micro- organisms. The methanolic extract of P. barbatus displayed a potent antibacterial activity against gram- positive bacteria including S. aureus (41) and a remark- able antifungal effect against C. albicans (42). Many diterpenoids isolated from several types of Plectranthus like P. fruticosus and P. saccatus were responsible for the antimicrobial effect (43–45). In our screening, the roots of the investigated P. hadiensis exhibited more antimi- crobial and antioxidant activity than the leaves. There are possibilities that similar or related compounds are present in the methanolic extract, which may be res- ponsible for the strong antibacterial and antioxidant effect. Besides the investigation of the leaves and roots of Tragia involucrata for their potential effect in wound healing, no reports about T. pungens or other species of this genus exist. T. involucrata displayed a high anti- bacterial effect against different bacterial strains espe- cially S. aureus (46,47). In comparison with the fact that no reports about V. bottae for any activity were found, V. macrurum and V. sinuatum showed antimicrobial activity (48,49). The presence of saponins and irridoids as major components could be the reason for the antimicrobial activity (50,51). Our phytochemical investigation showed the presence of saponins, iridoids and flavonoids, which may be responsible for the moderate antimicrobial and noteworthy antioxidant activities. In conclusion, the results obtained in the present study are in agreement to a certain degree with the traditional uses of the plants estimated. The obtained results could form a good basis for selection of plant species for further investigation in the potential discovery of new natural bioactive compounds. A. fruticosa, C. pseudosinaica, D. viscosa, J. variegata, L. citriodora, P. hadiensis, T. pungens and V. bottae could be a source for antibacterial drugs against Gram-positive bacteria,