Abstract
The present study investigated the antibacterial activities of some seaweeds (Ulva lactuca, Laurencia optusa, and Turbinaria triquatra) collected from Red Sea coast-Egypt. Ethanol crude extracts of these macroalgae, as well as their fractions (oleic acid, palmitic acid, fucoxanthin and fucosterol which, extracted from T. triquatra) with five different concentrations were tested against most nosocomial bacteria, such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa (Urinary tract infection (UTI), Bacillus cereus, Bacillus subtilis and Staphylococcus aureus (Hospitalized infection) using disc diffusion method. The highest antibacterial activity of crude ethanol extract was observed in T. triquatra (brown seaweed) followed by L. optusa (red seaweed) whereas, green seaweed (U. lactuca) showed less antibacterial activity. All the seaweeds crude extracts and their fractions have shown moderate antibacterial activity <10 mm of zone of inhibition. Antibacterial activities of crude ethanol extraction and their fractions were directly proportional to its concentrations and varied with the type of bacterial microbe. Furthermore, these extracts showed high antibacterial activities particularly against Gram positive bacteria rather than Gram negative bacteria. Moreover, mixtures of oleic and palmitic acids (1: 1) recorded the highest antibacterial activity against all tested bacteria followed by fucoxanthin, fucosterol, oleic acid, palmitic acid and crude ethanol extracts of algae. Scanning electron microscope showed that mixtures of oleic and palmitic acids caused plasmolysis and reduction in cell size of E. coli. This study reveals that the algal extracts and their fractions especially mixture of oleic and palmitic acids can be used as safe natural drug for treating resistance nosocomial bacterial infection if compared with the different chemical synthetic drugs.

Key words: Antibacterial, macroalgae, Turbinaria triquatra, Ulva lactuca, Laurencia optusa, bacterial nosocomial infection