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.

Automated summary

Although there are many opportunities to use microalgae as antimicrobial agents, more efforts are needed to move beyond the characterization phase and advance them to the biotechnology phase. One challenge is the ability of microalgae to synthesize biologically active secondary metabolites in response to environmental triggers. A rigorous scientific approach is required to identify potential microalgae strains with good antimicrobial activity and develop them as antimicrobial agents. The commonly used disc diffusion assay for screening antimicrobial activity in microalgae is problematic and prone to producing false-positive and false-negative results. Quantitative minimum inhibitory concentration (MIC) values obtained through assays like the microdilution broth assay are more reliable and allow for result comparison among research groups. This review compiled a dataset of published MIC values for microalgae, with Cyanobacteria and Chlorophyta being the most well-represented phyla. Factors influencing antimicrobial activity, such as test microorganisms, microalgae taxonomy, extraction solvents, and growth phase at harvest, were assessed using this data. The activity was categorized as good if the MIC values were <1 mg/mL, moderate if the MIC values were 1-8 mg/mL, and weak if the MIC values were >8.0 mg/mL. Areas requiring further research, including screening a greater diversity of species, reporting negative results, testing culture supernatants for activity, exploring synergistic effects, and identifying antimicrobial compounds in Chlorophyta, were discussed. The successful development and commercialization of microalgae antimicrobial agents will likely increase as more microalgae are screened and compounds are identified.