Abstract
The present study was performed to investigate the antimicrobial activity of water, ethanol or chloroform extracts of Zingiber officinale , Ocimum basilicum leaves and flowers and Taraxacum officinale leaves and roots. These extracts were tested in vitro against bacteria (Escherichia coli, CGSC1885, Escherichia coli, CGSC2463 and Bacillus megaterium), fungi (Aspergillus flavus and Rhizopus nigricans) and Candida albicans by disc diffusion method. The effect of extracts on bacteria was studied using transmission electron microscopy. The results indicated that Zingiber officinale, Ocimum basilicum flowers extracts (water, ethanol, and chloroform), exhibited an antimicrobial activity against the tested bacterial species. While the ethanol extract of Ocimum basilicum leaves and chloroform extract of Taraxacum officinale leaves and roots exhibited antimicrobial activity against Bacillus megaterium only. On the other hand, all extracts of the used plants being had no effect on Aspergillus flavus, Rhizopus nigricans, and Candida albicans. The cells of the bacteria, treated and untreated with the plants extracts, were examined by transmission electron microscope(TEM), it was observed that the size and ultrastructure of the treated cells by Zingiber officinale and Ocimum basilicum flowers extracts were affected compared with the control treatment. The ethanol and water extracts of Zingiber officinale and Ocimum basilicum flowers extracts (ethanol, water, and chloroform) degraded the cell wall, plasma cell membrane disintegrate and the cytoplasme leaches out of the cell. The results of the phytochemical studied revealed the presence of polyphenols, flavonoids, and terpenoids in water, ethanol, or chloroform extracts of Zingiber officinale and Ocimum basilicum flowers extract in water and the concentration of this compounds differ according to the the solvnt.

Key words: Antimicrobial activity, plant extract, Disc diffusion method, TEM, phytochemical analysis