DENISA BATIR MARIN, OANA CIOANCA, MIHAI APOSTU, CRISTINA GABRIELA TUCHILUS, CORNELIA MIRCEA, SILVIA ROBU, DANA TUTUNARU, ANDREIA CORCIOVA, MONICA HANCIANU THE COMPARATIVE STUDY OF EQUISETUM PRATENSE, E. SYLVATICUM, E. TELMATEIA: ACCUMULATION OF SILICON, ANTIOXIDANT AND ANTIMICROBIAL SCREENING The objective of the current study is represented by the determination of silica and a phytochemical screening of phenolic derivates of some Equisetum species. The antioxidant and antimicrobial activity for Equisetum pratense Ehrh.,, Equisetum sylvaticum L. and Equisetum telmateia Ehrh. (sin. Equisetum maximum Lam.) were also investigated. The concentration of silicon (Si) in plants was determined by the spectrophotometric method using previous treatment with NaOH 50% both for the stem and the nodal branches <1>. Results obtained varied from 95.12 to 162.10 SiO2 mg/g dry plant which represents 4.44% to 7.58% Si/100g dry sample. Two types of total extracts were obtained using different solvents and were subjected to qualitative and quantitative chemical analysis considering total phenolic content <2>. The highest concentration of investigated compounds was found in the methanolic extract, E. sylvaticum, 196.5mg/g dry sample. Antioxidant activity was monitored spectrophotometrically and expressed in terms of IC50 (µg/mL) <3>. Values gathered ranged from 261.7 to 429.5 µg/mL. The highest capacity to neutralized DPPH radicals was found in E. sylvaticum. In vitro antimicrobial activity was determined using difusimetric method <4>. Testing was performed on four microorganisms: three strains of bacteria and one species of fungi. Different effects were noticed against the bacteria, furthermore the methanol extract appeared to be most efficient. All extracts showed significand antimicrobial activity against Staphylococcus aureus (ATCC 25923) and Candida albicans (ATCC 90028) and weak to no activity against Pseudomonas aeruginosa (ATCC 27853) and Escherichia coli (ATCC 25922).