Activity Against Human Pathogens of Two Polyunsaturated Aldehydes and Pheophorbide a

Microalgae are extremely diverse photosynthetic organisms, adapted to live in different habitat conditions, from freshwater to marine environments. This adaptability is also associated with the ability to produce several metabolites. Polyunsaturated aldehydes (PUAs), first identified in 1999 in Thalassiosira gravida and Skeletonema costatum, are known to influence the development of their predators, having teratogenic effects and blocking their development. PUAs have shown several activities, such as antitumor, antimicrobial and antiparasite. Another relevant compound is pheophorbide a (PPBa), a chlorophyll degradation product, which has previously shown properties useful to be considered as a photosensitizer in photodynamic therapy, demonstrating cytotoxic effects on various tumor cell lines. It has also been shown to have activity against some bacteria and fungi. Considering the growing problem of multi-antibiotic resistance of human pathogenic bacteria and the increasing market demand for new drugs, the aim of our work was to screen two PUAs, i. e., 2,4-octadienal and trans,trans-2,4-decadienal, and PPBa against a panel of human pathogenic bacteria and fungi: Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Candida albicans and Aspergillus fumigatus. The antimicrobial activity was evaluated through MIC (Minimum Inhibitory Concentration) and MFC/MBC (Minimum Fungicidal/Bactericidal Concentration), demonstrating that the two PUAs had a greater antimicrobial activity than PPBa on both bacteria and fungi, except for P. aeruginosa, where the antimicrobial activity was low. The compound 2,4-Octadienal showed extremely high antifungal activity, especially against the fungus A. fumigatus, where the MIC and MFC were 0.001 µL/mL and 0.004 µL/mL, respectively. These results are shedding light on the antimicrobial activity of microalgal compounds and their possible applications for different human infection diseases.