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B. Adebayo-Tayo et al. Heliyon 5 (2019) e02502 average of 0.000 nm and 558.1 nm with a polydispersity index of 0.580 (Fig. 6). 3.7. Energy dispersive X-ray analysis of the OsSNPs The weight percentage of silver was 0.0132 from the EDX spectrum of OsSNPs (Fig. 7). The EDX analysis revealed strong signals in the silver region and confirmed the formation of silver nanoparticles. The micrograph showed that the biosynthesized silver nanoparticles are spherical in shape and the 3D-image revealed that the size of the nanoparticles is 10 nm. Raza et al. [38] reported the effect of size and shape of silver nanoparticles on the antibacterial activity of the silver nanoparticles and it was stated that small size spherical silver nano- particles are more active against microorganisms than larger spherical silver nanoparticles likewise that spherical silver nanoparticles are more effective against microbes than triangular shaped silver nanoparticles. The behavioural pattern of the silver nanoparticles biosynthesized from Oscillatoria sp. methanol extract when subjected to high tempera- ture. It can be generally attributed to the evaporation of water and organic components. The thermograph showed the relative weight of the nanoparticles with respect to heat. The biosynthesized OsSNPs was able to withstand heat to a certain extent before the significant weight lost. The DLS determines the size of particles and their distribution in physiological solutions [39]. Some distribution at lower range of particle size indicates that the synthesized particles are also in lower range of particle size. The size of biosynthesized SNPs obtained from DLS in this work is larger than SEM and this is in agreement with the report of Zhang et al. [40]. It was reported that the larger size of DLS than SEM is as a result of Brownian motion influence. There were other peaks for other elements suggesting that they are mixed precipitates present in the methanol extract of Oscillatoria sp. This agrees with the work of Khan et al. [41] who also confirmed SNPs for- mation in plant extract and observed the presence of other elements in the plant (Ziziphus nummularia) extract used in biosynthesis of SNPs. 3.8. Antibacterial potential of the OsSNPs The antibacterial activity of the biosynthesized OsSNPs was tested against seven clinical pathogenic bacteria. All the test pathogens were susceptible to the nanoparticle as shown Fig. 8. At 24 and 48 h Bacillus cereus and Citrobacter sp. had the highest susceptibility with zone of in- hibition 16.0 mm and 18.0 mm respectively. At 72 h Escherichia coli ATCC 35218 had the highest susceptibility to OsSNPs with 20.00 mm zone of inhibition while Bacillus cereus was least susceptible to the OsSNPs with lowest zone of inhibition (6.00 mm). Reduction in the zone of inhibition after 48 h may be due to production of enzyme by Escher- ichia coli ATCC 35218. The isolated has been reported as a beta lactamase producing strains [29]. Osc. sp. extract had antibacterial activity against the pathogen, the antibacterial activity ranged from 13.0 – 24.0 mm. The Fig. 6. Particle size distribution of the biosynthesized SNPs by DLS. highest activity was against P. aeruginosa. The extract exhibited higher antibacterial activity against some of the pathogens compared to the nanoparticles and AgNO3 solution AgNO3 solution had antibacterial ac- tivity against the test pathogens. The antibacterial activity ranged from 6.0 – 12.0 mm, the highest activity was against E. coli 35218. Fig. 9 shows the antibacterial activity of the nanoparticles on the test pathogen on plates. 3.9. Antibiofilm activity of the OsSNPs 3.9.1. Visual observation of biofilm formation The biofilm formation was assessed visually by observing formation of ring at the bottom of the test tube and at the meniscus level of the test tubes after staining with the crystal violet. 90 % of the test pathogens were biofilm formers (Table 1). 3.9.2. Antibiofilm potential of the OsSNPs The antibiofilm activity of the biosynthesized silver nanoparticles shows reduction in the biofilm formation by all test pathogens that were used thus implied that the biosynthesized silver nanoparticles is active against bacterial biofilm. The highest inhibition was against Pseudomonas aeruginosa ATCC 27853 while the lowest inhibition was against Cit- robacter sp. (Fig. 10). The OsSNPs showed more activity than silver nitrate solution which was used as control. The mechanism of antibacterial activity of silver nanoparticles is not well understood but some mechanisms have been proposed which include the release of silver ions when silver nano- particles come in contact with bacterial cell, these ions prevent DNA replication in such bacteria by deactivating the synthesis of enzymes that take part in DNA synthesis and ATP synthesis [15, 42]. Rai et al. [43] reported that silver nanoparticles can act as potential antimicrobial agents against different microorganisms. Raza et al. [44] reported that the antibacterial efficacy of silver nanoparticles is size and shape dependent while working on silver nanoparticles of different sizes and shapes. The antibacterial activity of silver nanoparticles from this research showed that Escherichia coli ATCC 25318 was more susceptible to the silver nanoparticles while Bacillus sp. was least susceptible to the biosynthesized silver nanoparticles. The result obtained from this research showed that the biosynthesized SNP is effective against all the selected microorganisms used in this experiment. Reduction in the zone of inhibition after 48 h may be due to production of enzyme by Escher- ichia coli ATCC 35218. The isolated has been reported as a beta lactamase producing strains [29]. The inhibition of biofilm formation by nanoparticles has been re- ported previously by Lee et al. [45], Seil and Webster, [46]. This research reports the biofilm inhibition of the biosynthesized silver nanoparticles with the highest inhibition observed in Pseudomonas aeruginosa ATCC 27853 and the least inhibition in Citrobacter sp. Lee et al. [45] in their research reported the inhibition of formation of biofilm of Pseudomonas aeruginosa by zinc oxide nanoparticles dose dependently. Vijayan et al. [47] also reported reduction in the formation of biofilm by silver nano- particles. Here we report the reduction in biofilm formation of selected gram positive and gram negative bacteria by silver nanoparticles bio- synthesized using methanol extract of Oscillatoria sp. 3.10. Cytotoxicity of OsSNPs The Artemia cytotoxicity (Brine Shrimp Assay) is one of the reliable methods to screen the cytotoxicity of products [44]. It was observed that the increase in mortality rate is dose dependent (Fig. 11). From the result, it was observed that the increase in mortality rate is dose dependent and this is in agreement with the work of Vijayan et al. [47] who also reported same thing while working on silver nanoparticles synthesized from the extract of seaweed Turbinaria conoides. Arulvasu et al. [48] in their work on the toxicity effect of silver nanoparticles in brine shrimp Artemia reported that Artemia possesses nonselective filter 5PDF Image | Green synthesis of silver nanoparticle Oscillatoria extract
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