Background: The incident of antibacterial resistance is increasing rapidly. Seaweeds, the marine macroalgae that are 
rich in bioactive compounds have the potential to be applied as an antimicrobial agent.
Objective: The objective was to assess the potential of brown seaweed aqueous extracts (Sargassum polycystum and Padina australis) and commercial pure compounds (alginic acid and fucoidan) as antimicrobial agent toward Gram-negative bacteria such as Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa as well as Gram-positive bacteria, Staphylococcus aureus.
Methods: Powdered brown seaweeds were macerated with distilled water and followed by filtration. The aqueous extracts 
were recovered by ethanol precipitation, oven-dried, and stored at 4°C. Aqueous extracts were examined for their qualitative phytochemical content and antibacterial susceptibility test using Kirby–Bauer method.
Results: S. polycystum was reported for higher extraction yield (7.00%) than P. australis (1.40%). The presence of steroid, 
phytosterols, and phenols was observed in S. polycystum extract while steroid and phytosterols were observed in P. australis extract. In antibacterial susceptibility test, there was no inhibition shown by both seaweed extracts against the four bacteria tested. Fucoidan (0.50 mol/L) and alginic acid (0.50 mol/L) were observed to have antibacterial property against S. aureus (with inhibition zone of 7.43±0.17 mm and 7.45±0.14 mm, respectively).
Conclusion: The aqueous extracts of S. polycystum and P. australis have shown low antibacterial activity, as compared to fucoidan and alginic acid. Further studies on the mode of antibacterial activity possessed by fucoidan and alginic acid will be carried out.

Kamenarska, Z.; Serkedjieva, J.; Najdenski, H.; Stefanov, K.; Tsvetkova, I.; Dimitrova-Konaklieva, S.; Popov, S. Antibacterial, Antiviral, and Cytotoxic Activities of Some Red and Brown Seaweeds From the Black Sea. Bot. Mar., 2009, 52 (1), 80-6.

Ismail, A. Rumpai laut Malaysia. Malaysia: Dewan Bahasa dan Pustaka, Kementerian Pendidikan; 1995.

Jaswir, I.; Noviendri, D.; Salleh, H.M.; Taher, M.; Miyashita, K. Isolation of Fucoxanthin and Fatty Acids Analysis of Padina australis and Cytotoxic Effect of Fucoxanthin on Human Lung Cancer (H1299) Cell Lines. Afr. J. Biotechnol., 2013, 10(81), 18855-62.

Shibu, A.; Dhanam, S. Investigation of in vitro Antibacterial Evaluation of Some Marine Algae Collected from Gulf of Myanmar, Tamil Nadu. Int. J. Recent Sci. Res., 2013, 4(4), 376-80.

Gupta, S.; Abu-Ghannam, N. Bioactive Potential and Possible Health Effects of Edible Brown Seaweeds. Trends Food Sci. Technol., 2011, 22(6), 315-26.

Chotigeat, W.; Tongsupa, S.; Supamataya, K.; Phongdara, A. Effect of Fucoidan on Disease Resistance of Black Tiger Shrimp. Aquaculture, 2004, 233, 23-30.

De Souza, M.C.R.; Marques, C.T.; Dore, C.M.G.; Da-Silva, F.R.F.; Rocha, H.A.O.; Leite, E.L. Antioxidant Activities of Sulfated Polysaccharides From Brown and Red Seaweeds. J. Appl. Phycol., 2007, 19, 153-60.

Soler, P.; González-Sanz, R.; Bleda, M.J.; Hernández, G.; Echeíta, A.; Usera, M.A. Antimicrobial Resistance in Nontyphoidal Salmonella from Human Sources, Spain, 2001-2003. J. Antimicrob. Chemother., 2006, 58 (2), 310-4.

Wu, W.; Jin, Y.; Bai, F.; Jin, S. Part 6: Disseminating bacterial infections. Pseudomonas aeruginosa. In: Tang, Y.W.; editor. Molecular Medical Microbiology. 2nd ed. UK: Academic Press; 2014. p. 753-67.

World Health Organization. Antimicrobial Resistance, Key Facts, Present Situations; 2018. Available from: https://www.who.int/en/news-room/fact-sheets/detail/antimicrobial-resistance.

Centers for Disease Control and Prevention. Active Bacterial Core Surveillance Report, Emerging Infections Program Network, 2018. p. 35-51.

de Aragão, A.P.; de Oliveira, T.M.; Quelemes, P.V.; Perfeito, M.L.G.; Araújo, M.C.; Santiago, J. de A.S.; da Silva, D.A. Green Synthesis of Silver Nanoparticles Using the Seaweed Gracilaria birdiae and their Antibacterial Activity. Arab. J. Chem., 2016, DOI: 10.1016/j.arabjc.2016.04.014.

Tiwari, P. Phytochemical Screening and Extraction A Review. Int. Pharm. Sci., 2011, 1(1), 98-106.

Gul, R.; Jan, S.U.; Faridullah, S.; Sherani, S.; Jahan, N. Preliminary Phytochemical Screening, Quantitative Analysis of Alkaloids, and Antioxidant Activity of Crude Plant Extracts from Ephedra intermedia Indigenous to Balochistan. Sci. World J., 2017, 2017, 1-7.

Christabel, J.; Lipton, A.P.; Aishwarya, M.S.; Sarika, A.R.; Udayakumar, A. Antibacterial Activity of Aqueous Extract From Selected Macro Algae of Southwest Coast of India. Seaweed Res Util., 2011, 33(1-2), 67-75.

Elsie, B.H.; DhanaRajan, M.S. Evaluation of Antimicrobial Activity and Phytochemical Screening of Gelidium acerosa. J. Pharm. Sci. Res., 2010, 2(11), 704-7.

Vijayabaskar, P.; Vaseela, N.; Thirumaran, G. Potential Antibacterial and Antioxidant Properties of a Sulfated Polysaccharide from the Brown Marine Algae Sargassum swartzii. Chin. J. Nat. Med., 2012, 6, 421-8.

Kanimozhi, A.S.; Johnson, M.; Malar, R.T. Phytochemical Composition of Sargassum polycystum C. Agard and Sargassum duplicatum J. Agardh. Int. J. Pharm. Pharm. Sci., 2015, 7, 393-7.

Nurhidayah, N.S. Antimicrobial Potential of Padina australis and Sargassum polycystum against Respiratory Infections Causing Bacteria Bachelor Dissertation, 2018.

Melpha, Y.; Manchu, N.; James, J.E. Phytochemical Evaluation of Two Brown Seaweeds from Muttom and Rasthacaud Coasts of Tamil Nadu, India. J. Chem. Pharm. Res., 2014, 6(10), 566-9.

Maharany, F.; Nurjanah, N.; Suwandi, R.; Anwar, E.; Hidayat, T. Bioactive Compounds of Seaweed Padina australis and Eucheuma cottonii as Sunscreen Raw Materials. J. Pengolahan Hasil Perikanan Indonesia, 2017, 20(1), 10.

Chong, C.W.; Hii, S.L.; Wong, C.L. Antibacterial Activity of Sargassum polycystum C. Agardh and Padina australis Hauck (Phaeophyceae). Afr. J. Biotechnol., 2011, 10(64), 14125-31.

Marudhupandi, T.; Kumar, T.T.A. Antibacterial Effect of Fucoidan from Sargassum wightii against the Chosen Human Bacterial Pathogens. Int. Curr. Pharm. J., 2013, 2 (10), 156-8.