Silver nanoparticles (AgNPs) were biosynthesized using the cell free supernatant of putative probiotic Lactobacillus paracasei A26. Several biological activities of biogenic AgNPs were investigated in respect to in vitro anti-oxidant and anti-tumor potentials. Anti-oxidant potentials were screened in terms of free radical scavenging activity against two free radicals, 2, 2-Diphenyl-1-picrylhydrazyl (DPPH) and resazurin dye. AgNPs exhibited a potent scavenging activity against resazurin dye (91±0.046%) with an EC50 concentration of 146.823 µg/ml, while scavenging of DPPH was significantly (P≤0.05) reduced to 72.330±0.114% using a higher EC50 concentration of 176.12 µg/ml. The anti-tumor potentials of biogenic AgNPs were studied in relation to the cytotoxicity against two human breast cancer cell lines (CAL-51 and MCF7), using crystal violet dye assay. The viability of AgNPs-treated cancerous cells was significantly decreased in a time- and concentration manner, as compared to insignificant cytotoxic effects against the normal cell line. However, the anti-proliferative activity of AgNPs did not exceed the value of 63.85±0.019% in both cancer cell lines. CAL-51 cells were the most sensitive to the introduced AgNPs, with a maximum decrease in viability of 49.889±0.021% being reached using an IC50 value of 98.65µg/ml for 48h exposure time. The inhibition percentage was increased to 60.13±0.005% when the used IC50 value was significantly declined to 40.73µg/ml with an exposure time expanded to 72h. MCF7 cells showed lower sensitivity than CAL-51 cells, but with a similar inhibition trend of 59.523±0.01% with an IC50 concentration of 66.54 µg/ml for 48h which was increased to 63.857±0.019% when the IC50 was reduced to 62.63 µg/ml and the exposure time expanded to 72h. The morphological changes of AgNPs-treated cells were apparent at 72h exposure time, with cells showing apoptotic-like features such as shrinkage and losing of regular fusiform shape. Moreover, cells became detached to surfaces and from each other.
