Microalgae have been explored for their bioactive compounds with promising applications encompassing
antibacterial, antiviral, antifungal and antialgal activities. Considering the present status of widely used treatment therapies and their limitations to tackle their adverse effects, the application of bioactive compounds derived from algae will prove beneficial and much more effective as compared with traditional treatment methods. Due to the emerging infectious diseases, viral infections (epidemic and pandemic) and raise in antibiotic resistant bacteria, there is an urgent need for development of alternative treatment therapies against infectious diseases. Present work provides a brief introduction about the algal bioactive compounds and their activities against various pathogens.
Keywords: Microalgae; Antibacterial; Antifungal; Antiviral
Algae has wide span of ecosystems contributes to the innumerable antimicrobial compounds have been identified in miceoalgae as well as macroalgae . More than 18,000 new compounds have been isolated from marine sources, yet majority of them have not yet been obtained nor characterized . Therefore, microalgae represent a unique opportunity to discover novel metabolites. The rate of finding metabolites already obtained from other biological sources is less in microalgae as compared with other microorganisms . Due to their metabolic plasticity under stressed vs. nonstressed conditions microalgae possess the extra advantage of triggering secondary metabolism . As microalgae where potentially explored only after 1950s, they were not considered previously for therapeutic purposes. Extensive search is presently undergoing to find novel therapeutically useful agents [5-7]. Microalgae have meanwhile been found to produce antibiotics. A large number of microalgal extracts and/or extracellular products have proven antibacterial, antifungal, antiprotozoal and antiplasmodial [6-13]. Efforts to identify the compounds directly responsible for those antimicrobial features have been made, but are still embryonic.
We have been working with algae like Chlorella and Chlamydomonas (Figure 1) isolated, maintained and extracted as described by Salem et al. . These extracts were later used for antibacterial assay and determination of minimum inhibition concentration (MIC). Antibacterial activity of algal extracts determines the MIC of algae used in this study in vitro .
Algal cell-free extracts are already being tested [15-17]. Our aim is to provide information about the recent trends in the discovery of bioactive compounds derived from algae which have shown their potential as antimicrobial agents. We have briefly summarized the recent works carried out by the researchers globally in the field of algal antimicrobial activities.
Antimicrobial activities of algal extracts
Antibacterial activity of algae: The needs for development of alternative antibiotic agent were investigated since the emergence of antibiotic resistant microbes. Due to the emergence of drugresistant pathogens they endanger people in affluent, industrial societies like the United States, as well as in less-developed nations. Examples of clinically important microbes that include Streptococcus pneumoniae, Staphylococcus aureus, Escherichia coli, Salmonella or E. coli and infections transmitted healthcare malpractices enterococci, Acinetobacter baumanii, Pseudomonas aeruginosa, and Klebsiella spp . The development in the field of algal therapeutic research has made it possible by their bioactive compounds which have been found effective against most of the pathogens (Figures 1 and 2). The list of some of the algal bioactive compounds is summarized in the Table 1 [19-24].
|Algal species||Extract source||Target bacteria||Reference|
|Pithophoraoedogonium||Ethanol extract||Salmonella, Staphylococcus sp., 4978|||
Diethylether Dichloromethane Ethanol
|Gram negative and Gram positive bacteria|||
|Sargassumwightii,Chaetomorphalinum, Padinagymnospora.||Acetone,methonol||P. aeruginosa(ATCC27853),
Erwiniaamylovora(MTCC2760) (E. amylovora), Enterobacteraerogenes (MTCC111) (E. aerogenes), Proteus vulgaris(MTCC1771) (P. vulgaris), Klebsiellapneumonia(ATCC15380) (K. pneumonia) and E. coli(ATCC25922). gram-positive bacterial strains were Methicillin resistant S. aureus,
|Asparagopsistaxiformis||Ethanol extract||Vibrio alginolyticus, Vibrio vulnificus and Aeromonassalmonicida subsp. salmonicidaPhotobacteriumdamselaesubsp. Damselaeand Photobacteriumdamselae subsp. piscicida, Salmonella sp., Vibrio cholerae, Vibrio harveyi and Vibrio parahaemolyticus|||
|Chlorococcumhumicola||Bioactive compounds||Escherichia coli, Pseudomonas aeruginosa, Salmonella typhimurium, Klebsiellapneumoniae, Vibreocholerae, Staphylococcus aureus, Bacillus subtilis, Candida albicans, Aspergillusniger and Aspergillusflavus.|||
|Gloeocapsasp. Synechocystissp. Anabaenasp. Aphanizomenonflos-aquae. Nostocsp. Nostocentophytum, Nostocmuscorum, Scytonemaocellatum, Arthrospirafusiformis(Voronich), Scenedesmusobliquus, Coelastrellasp. Chlorellasp. Rhodellaviolacea, Porphyridiumcruentum(AG.) NAG||Ethanol extract||Staphylococcus aureus209, Streptococcuspyogenes981, Bacilluscereus2421 Escherichia
coli3702, Pseudomonasaeruginosa1396, Salmonella
typhimurium123, and Yersiniaenterocolitica623
Table 1: Antibacterial activity of selected compounds from microalgae.
Antiviral activity of algae: The viruses have been the cause of mass epidemic and pandemic outbreaks of potentially harmful and deadly diseases like influenza, hepatitis, etc. Due to the unavailability of proper treatment facilities and precautionary measures they have been causing a great panic worldwide. Considering the present situation the discovery of antiviral compounds which were derived from algal bioactive compounds provide us a great relief. These compounds, which are tabulated in Table 2 [25-28] has a great prospective in the future.
|Algal species||Extract source||Target Virus||Reference|
virus type 1
|Sulfated polysaccharide||Influenza virus|||
|Naviculadirecta||Polysaccharide||HSV1 & 2,
Influenza A virus
Table 2: Antiviral activities of selected compounds from microalgae.
A number of infectious diseases caused by viruses have re-emerged in recent years, new antiviral measures are necessary for those who are not exposed to them previously. Due to this microalgae have received a strong attention to be explored for potential antiviral agents .
Antifungal activity of algae: The study of resistance to antifungal agents has lagged far behind that of antibacterial resistance likely because fungi were not recognized as important pathogens [30,31]. The associated increase in fungal infections prompted search for newer and safer agents to combat fungal infections  and a few noteworthy results encompassing microalgae are listed in Table 3 [19,24,33].
|Algal species||Extract source||Target fungi||Reference|
|Pithophoraoedogonium||Ethanol extract||Penicilliumviridicatum1101, Fusariumsolini1127|||
|Gloeocapsa sp.||Exopolysaccharides||Candida albicans|||
|Haematococcuspluvialis||Butanoic acid and methyl lactate||Candida albicans|||
Table 3: Antifungal activities of selected compounds from microalgae.
Antialgal activity of algae: Inhibitory phenomena between microalgal cells have been reported in the past; Bagchi et al.  originally proposed that natural algaecides could effectively be applied in control of toxic algal blooms like Isochrysis galbana from cell-free filtrates Dunaliella salina, Platymonas elliptica, C. vulgaris, Chaetoceros muelleri, Chlorella gracilis, Nitzschia closterium and P. tricornutum . However, Pratt  was the first to report that growth of C. vulgaris was depressed by a compound (chlorellin) that was produced and excreted into the medium - and several other extracellular metabolites able to inhibit their own growth and the growth of other species have meanwhile been reported .
Antiprotozoal and Antiplasmodial activity of algae: The antiprotozoal activities algal extracts have recently been discovered against Trypanosoma brucei rhodesiense, Trypanosoma cruziand Leishmania donovaniand were found effective. The development of antiprotozoal algal extracts may prove effective in controlling various protozoan diseases and their preventive measures . The crude seaweed extracts from green marine algae Cladophora rupestris, Codium fragile ssp. tomentosoides, Ulvaintestinalisand Ulva lactucahave shown anti protozoan activity against Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani . Ciau et al. studied the antiprotozoal activity of brown alga Lobophora variegataagainst Giardia intestinalis, Entamoeba histolyticaand Trichomonas vaginalis. They have extracted antiprotozoal compound by chloroform, the major compounds included -O-palmitoyl-2-O-myristoyl-3-O-(6′′′-sulfo-a-D-quinovopyranosyl)-glycerol;1,2-di-O-palmitoyl-3-O-(6′′′-sulfo-a-D-quinovopyranosyl)-glycerol and a new compound identified as 1-O-palmitoyl-2-O-oleoyl-3-O-(6′′′-sulfo-a-Dquinovopyranosyl)-glycerol . The ethanolic extracts of freshwater macrophytes Potamogeton perfoliatus, Ranunculus tricophyllusand Cladophora glomerataas well asmarine macroalgae Dictyota dichotoma, Halopteris scoparia, Posidonia oceanica, Scinaia furcellata, Sargassum natansand Ulva lactucaare assayed for their in vitro antiprotozoal activity against Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovaniand Plasmodium falciparum .
Trypanosomiasis is one of the most important parasitic diseases worldwide. The undesirable side effects and low efficacy of classical trypanocidal drugs underline the necessity of the development of new drugs from natural products. Although marine algae have been recognized as attractive sources of known and novel bioactive compounds, very little research has been focused on antiprotozoal activity. Aqueous and organic extracts of Rhodophyta, Phaeophyta and Chlorophyta were evaluated for their antiprotozoal activity in vitro against Trypanosoma cruzi trypomastigotes. The organic extracts from Dictyota caribea, Lobophora variegata, Turbinaria turbinata Linnaeus, and Laurencia microcladia Kützing possess promising in vitro activity against T. cruzi trypomastigotes. Laurencia microcladia is effective against Artemia salina and the high cytotoxicity exhibited by T. turbinate is required to be investigated further .
Red alga from genus Chondria produces cyclic polysulfides, terpenoids, amino acids and amines. Domoic acid derivatives from Chondria armata show larvicidal and blood pressure lowering activity . The algal extracts have also been explores for their antiplasmodial activities, . The P. falciparum (Erythrocytic stages), T. cruzi (Trypomastigotes) and L. donovani (Axenic amastigotes) are growth inhibited with the ethanol and ethyl acetate extract of algae belonging to Chlorophyta, Heterokontophyta and Rhodophyta. Antimalarial leads from marine algae, four metabolites, sargaquinoic acid, sargahydroquinoic acid, sargaquinal and fucoxanthin, were isolated from the Sargassum heterophyllum. Fucoxanthin and sargaquinal showed good antiplasmodial activity toward a chloroquine-sensitive strain of Plasmodium falciparum  Ethylacetate (EtOAc) extract of Sargassum swartzii and Chondria dasyphylla were investigated for larvicidal activities in larvae of malaria vector Anopheles stephensi and the mortality rate of Anopheles stephensi was 96 and 95%, respectively . The endemic marine red alga Plocamium cornutum (Turner) Harvey show antiplasmodial activity in organic extracts. Interestingly, compounds bearing the 7-dichloromethyl substituent showed significantly higher antiplasmodial activity toward a chloroquine sensitive strain of Plasmodium falciparum .
We have screened the antibacterial activities of organic extracts of isolated culture of algal species and had evaluated them by agar well diffusion method. Methanol extract and ethyl acetate extract of algae were effective against few bacterial species including Staphylococcus spp. and E.coli. Methanol extracts were more effective as compared with ethyl acetate extract of algae. The antibacterial and antifungal activities were seen predominantly from the chlorella sp. as well as Chlamydomonas sp. Our work clearly summarizes the importance of microalgal extracts which have potential implication as antibacterial, antiviral, antifungal, antimicroalgal antiprotozoal as well as antiplasmodial agents. This information can prove very helpful in further research and discovery of new drugs. The work briefly explains the work carried out by various researchers, clearly elaborating the important implications of algal bioactive compounds for the application against infectious diseases and as an antimicrobial therapy.