Abstract: The microbial community associated with mangrove plant (Avicennia marina) in Safaga (Red Sea) was studied, the heterotrophs (TVC), Vibrio, Aeromonas and Staphylococcus counts in sea water were 56000, 200, 300 and 160 cfu mL-1, respectively. The mangrove stems harboured lower values and the roots harboured higher values. The dominant heterotrophs isolated from the roots and stems were: Bacillus, Vibrio, Aeromonas and Pseudomonas. Different extracts of the different parts of the plant (seeds, leaves, stems and roots) were applied on different bacterial pathogens such as: P. aeruginosa, V. fluvialis, V. vulnificus, S. faecalis, E. coli, S. aureus and B. subtilis. The chloroform extracts showed considerable activities against the different pathogens, while the activity of the ethanol extracts showed lower values. The chloroform seeds extracts inhibited the growth of all pathogens efficiently and recorded the highest activity unit (AU = 25.0) against the fish pathogen V. fluvialis. Chemical composition of the extract contained carbohydrates, proteins and lipids (2.58, 0.74 and 0.074 mg), respectively, in addition to flavonoids, triterpenoids, lignin and tannin (8.6, 3, 11 and 8%), respectively. The study extended to apply these extracts on Nile tilapia sp. (Oreochromis niloticus) aquaculture, 2.5 and 5 ml L-1 of the chloroform seeds extracts were applied, 5 ml L-1 showed satisfied results while the efficiency ranged from 64.1% in the second day to 79.4% in the six day.