Antimicrobial and anti-biofilm activity of silver nanoparticles biosynthesized with Cystoseira algae extracts

Antimicrobial resistance is an ever-growing global concern to public health with no clear or immediate solution. Silver nanoparticles (AgNPs) have long been proposed as efficient agents to fight the growing number of antibiotic-resistant strains. However, the synthesis of these particles is often linked to high costs and the use of toxic, hazardous chemicals, with environmental and health impact. In this study, we successfully produced AgNPs by green synthesis with the aid of the extract of two brown algae—Cystoseira baccata (CB) and Cystoseira tamariscifolia (CT)—and characterized their physico-chemical properties. The NPs produced in both cases (Ag@CB and Ag@CT) present similar sizes, with mean diameters of around 22 nm. The antioxidant activity of the extracts and the NPs was evaluated, with the extracts showing important antioxidant activity. The bacteriostatic and bactericidal properties of both Ag@CB and Ag@CT were tested and compared with gold NPs produced in the same algae extracts as previously reported. AgNPs demonstrated the strongest bacteriostatic and bactericidal properties, at concentrations as low as 2.16 µg/mL against Pseudomonas aeruginosa and Escherichia coli. Finally, the capacity of these samples to prevent the formation of biofilms characteristic of infections with a poorer outcome was assessed, obtaining similar results. This work points towards an alternative for the treatment of bacterial infections, even biofilm-inducing, with the possibility of minimizing the risk of drug resistance, albeit the necessary caution implied using metallic NPs. Open access funding provided by FCT|FCCN (b-on). This work was supported by the “Contrato-Programa” UIDB/04050/2020 funded by national funds through the FCT I.P., project ATLANTIDA— platform for the monitoring of the north Atlantic Ocean and tools for the sustainable exploitation of the marine resources”, operation NORTE-01–0145-FEDER-000040, co-financed by NORTE2020 through the European Regional Development Fund (ERDF), and by the Xunta de Galicia Ref.: ED431C 2018. MF (SFRH/BD/147819/2019) holds a scholarship from Fundação para a Ciência e a Tecnologia I.P. NGB thanks for her postdoctoral Margarita Salas grant to Universidade de Vigo and Ministerio de Ciencia, Innovación y Universidades (Spain). Raul Machado acknowledges FCT I.P. for funding within the Scientifc Employment Stimulus instrument (CEECIND/00526/2018).