Background: The escalating global threat of antimicrobial resistance underscores the critical need for novel antimicrobial agents. This study aimed to investigate the effect of different light spectra on the biosynthesis of bioactive compounds and the consequent antimicrobial efficacy of propanolic and butanolic extracts from the cyanobacterium Arthrospira platensis (Spirulina).
Materials and Methods: A. platensis was cultured for 46 days in Zarrouk›s medium under illumination with specific LED light spectra (white, yellow, red, blue, and green). The resulting algal biomass was subsequently extracted using propanol and butanol. The bioactive metabolites within these extracts were identified by gas chromatography-mass spectrometry. The antimicrobial activity of the extracts was evaluated against two gram-positive (Bacillus cereus, Staphylococcus aureus) and two gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacterial pathogens using the disk diffusion assay and broth microdilution to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC).
Results: Our results demonstrate that light quality significantly modulates metabolite production and antimicrobial potency. Green light irradiation markedly enhanced the production of known antimicrobial compounds, including bis(2-ethylhexyl) phthalate (DEHP) and phytol, and induced the most potent activity against B. cereus. The butanolic extract from green light-treated cultures exhibited exceptional efficacy against B. cereus (inhibition zone of 18.67 mm, MIC=8 µg/mL, MBC=16 µg/mL). Conversely, blue light treatment yielded a more diverse metabolome (137 compounds identified), with its propanolic extract showing only modest or no detectable activity against the tested Gram-negative pathogens (MIC≥1024 µg/mL for E. coli).
Conclusion: These findings establish light spectrum manipulation as a powerful, non-invasive strategy to selectively enhance the production of antimicrobial secondary metabolites in A. platensis, highlighting its potential as a tunable source for natural antibiotic discovery.