Effect of growth conditions and extraction solvents on enhancement of antimicrobial activity of the microalgae Chlorella vulgaris

Owing to the alarming increase of drug resistance among microbial pathogens, many efforts have been made to discover and characterize new antimicrobial agents (1, 2). Aquatic organisms including bacteria, algae, sponges, aquatic weeds, etc. produce a large number of bioactive compounds with antimicrobial, anticancer and antioxidant activity. The organisms produce these compounds to protect themselves against biotic and abiotic factors (3). Algae comprise of a heterogeneous group of organisms which are divided into microalgae and macroalgae, based on their size. These organisms produce a high variety of secondary metabolites in order to adapt themselves to the environmental conditions and/or to protect against predation, herbivory and to compete for space (4). In this regard, various materials such as amino acids, terpenoids, phlorotannins, alkanes, halogenated ketones, steroid compounds, cyclic polysulfides, fatty acids, acrylic acid, phenols, etc. are obtained from algae, some of which have good antimicrobial activity and could be employed as pharmaceutical agents (5, 6). According to the source of carbon, microalgae can be classified as autotrophs, mixotrophs and heterotrophs. Autotrophic microalgae require only inorganic compounds such as CO2 as a carbon source while, heterotrophic microalgae cannot fix carbon and use organic carbon for growth. Some photosynthetic Abstract Regarding increasing rate of drug resistance among microbial pathogens, a global search to find new antimicrobial agents from natural compounds with fewer side effects has been considered by many researchers worldwide. Bioactive compounds with good antimicrobial activity have been isolated from different algae and cyanobacteria. The current study was performed to determine antimicrobial potential and to characterize the effect of algal growth modes (autotrophic, heterotrophic and mixotrophic) and extraction solvents on antimicrobial activity of the microalgae Chlorella vulgaris against four bacterial and one fungal pathogens. C. vulgaris was grown under different growth conditions and the biomass was harvested. Different extracts were prepared using acetone, chloroform and ethyl acetate as extraction solvents and antimicrobial activity of the extracts was investigated against two Gram positive bacteria (Bacillus subtilis and Staphylococcus aureus), two Gram negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and a fungal strain (Candida albicans) using agar well diffusion assay. In addition, Minimal Inhibitory Concentration (MIC) of the extracts were determined. Moreover, the total oil content and lipid productivity of C. vulgaris grown under different modes were determined. The heterotrophic growth resulted in stronger antimicrobial activity compared to the other growth conditions. In addition, the highest antimicrobial activity was observed for chloroform mediated extract and extraction using acetone resulted in minimum antimicrobial activity. Moreover, heterotrophic and mixotrophic growth significantly increased the total lipid content and lipid productivity compared to the autotrophic growth. C. vulgaris exhibited good antimicrobial potential and the antimicrobial efficacy could be influenced by extraction solvents and growth conditions.