Bioprospecting for bioactive compounds in microalgae: Antimicrobial compounds.

While there are many opportunities to use microalgae as antimicrobial agents, little has been done to develop them beyond the characterization phase to the biotechnology phase. One challenge when screening microalgae for antimicrobial activity is their ability to synthesize biologically active secondary metabolites in response to environmental triggers. In order to identify potential strains with good antimicrobial activity and to advance the development of microalgae as antimicrobial agents, a rigorous scientific approach is required. Microalgae are most commonly screened for antimicrobial activity using the disc diffusion assay but this assay is problematic and produces false-positive and false-negative results. Quantitative minimum inhibitory concentration (MIC) values generated in assays such as the microdilution broth assay are more reproducible and enable comparison of results between research groups. For the present review, a dataset was compiled of published MIC values for microalgae. The Cyanobacteria and Chlorophyta were the best represented and other phyla were under represented. This data was used for assessment of factors influencing antimicrobial activity, including test microorganisms, microalgae taxonomy, different solvents for extraction and the growth phase at harvest. Activity was considered good if MIC values were < 1 mg/mL, moderate if MIC values were 1-8 mg/mL and weak with MIC >8.0 mg/mL. Areas requiring more research are discussed including screening a greater diversity of species in appropriate assays, reporting negative results, testing the culture supernatant for activity, synergistic effects and identifying antimicrobial compounds in the Chlorophyta. The potential for successful development and commercialization of microalgae antimicrobial agents will increase as more microalgae are screened and compounds identified.