Bioprospecting and LED-based spectral enhancement of antimicrobial activity of microalgae isolated from the west of Ireland

Antimicrobial screening programmes have shown that microalgae constitute a rich source of secondary metabolites exhibiting antimicrobial activity against human and aquaculture pathogens. In this study, extracts from 80 newly isolated marine and freshwater microalgae strains were tested for antimicrobial activity against 6 pathogens (Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Enterococcus faecalis, Staphylococcus aureus and Candida albicans). Principal Component Analysis and Hierarchical Clustering analyses (PCA-HC) of 4320 disc diffusion inhibition zones defined three main groups comprising strains with polar fractions inhibiting grampositive bacteria, strains which exhibited broad spectrum activity and strains with non-polar fractions inhibiting gram-positive bacteria and C. albicans. Minimum Inhibitory Concentration (MIC) values were determined for 23 promising strains, which returned an activity threshold >= 15 mm inhibition zone diameter by disc diffusion method for at least one pathogen. The relationship between spectral quality and antimicrobial activity was further investigated for 5 promising strains (diatom Stauroneis sp. LACW_24, haptophyte Prymnesium sp. DMGCW_41, and chlorophytes cf. Chlorococcum sp. DMGCW_43, cf. Micractinium sp. LACW_01 and Tetraselmis sp. LACW_06), indicating a significantly higher bioactivity under blue light for the chlorophytes and red light for the diatom Stauroneis sp. LACW_24. Polar fractions from cf. Chlorococcum sp. DMGCW_43 and Stauroneis sp. LACW_24 possessed broad spectrum antibacterial activity with MIC values of 157 mu g/mL and 875 mu g/mL against P. aeruginosa, 157 mu g/mL and 1000 mu g/mL against E. coli, 438 mu g/mL and 875 mu g/mL against B. subtilis and E. faecalis, 110 mu g/mL and 219 mu g/mL, against S. aureus, respectively. This screening programme has led to the identification of several strains with promising antimicrobial activity. In addition, the manipulation of LED-based spectral quality to enhance the bioactivity of antimicrobial components within several candidate strains is described for the first time.