The present study demonstrates the antibacterial activity of selected brown and green marine algae collected from Saudi Arabia Red Sea and Arabian Gulf. The methanolic and acetone extracts were tested against gram positive, gram negative bacteria and in an attempt to be used as an alternative to commonly used antibiotics. Both brown seaweed species and methanolic extracts were found to be active against gram positive than gram negative; however, acetone extract gave the highest inhibitory activity against sp. On the other hand, organic extract demonstrated higher antibacterial activity than the fresh extract but both extracts revealed decreased activity compared to extracts. methanolic extract showed an obvious effect on methicillin resistant (MRSA). The present work shows a comparable therapeutic potency of the tested seaweed members and extracts in treating human microbial pathogens to synthetic chemical antibiotics. A remarkable higher antioxidant DPPH free radical scavenging effect was recorded with sp. compared to sp. FTIR Infrared Spectrometer analysis together with the high performance liquid chromatography provided a detailed description of the possible functional constituents and the major chemical components present in marine macroalgae particularly in brown seaweeds to be mainly of phenolic nature to which the potent antimicrobial activity is being attributed.

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