Biogenic synthesis and characterization of silver nanoparticles and their effects against bloom-forming algae and synergistic effect with antibiotics against fish pathogenic bacteria

The present work evaluates the bacteriocidal and algicidal efficacy of nanosilver particles (AgNPs) synthesized through a green pathway by using the cyanobacterium Synechococcus elongatus. Further, the synergistic effects of the conjugates of selected antibiotics and biogenic silver nanoparticle were evaluated against three Gram-negative fish pathogens, viz. Aeromonas hydrophila ATCC-7966, Vibrio parahaemolyticus ATCC BAA-238 and Edwardsiella tarda ATCC-15947. The biogenic silver nanoparticles were polydispersed and showed a size range 71–201 nm with a zeta potential of −9.11 mV. The antibacterial activities of ampicillin, gentamicin, ofloxacin and ciprofloxacin exhibited a significant (P < 0.05) increase when conjugated with AgNPs against the selected bacterial strains. The highest synergistic effect of conjugates of AgNPs with gentamicin and ampicillin was observed against E. tarda. The biogenic silver nanoparticles exhibited a considerable inhibition of growth and decrease in photosynthetic pigments in a bloom-forming alga (Microcystis aeruginosa) and biofouling alga (Phormidium sp). This report aims to establish that biogenic silver nanoparticles are effective against water-borne fish pathogens and their efficacy is comparable with silver nanoparticles synthesized through chemical-based pathways. Since most of the reports on antibacterial properties of silver nanoparticles are on human pathogens, this report will provide baseline data for aquatic pathogens which result in a considerable loss of fish production due to disease outbreak.