Evaluation of Antibiofilm and Antibiotic Synergistic Activities of Silver Nanoparticles Against Some Common Bacterial Pathogens

Introduction: Nowadays silver nanoparticles (AgNPs) are used as antimicrobials due to their wellknown biochemical properties. The current study was planned to search the antimicrobial activities of AgNPs alone and in combination with common antibiotics against some field isolates of common bacterial pathogens. Methods: Standard strains of Escherichia coli, Salmonella enteritidis, Staphylococcus aureus and Streptococcus agalactiae, in addition to 9 field isolates for each strain (totally 40 strains, 10 for each) were used. Macrodilution method for determination of minimum inhibitory concentration (MIC) of AgNPs and antibiotics against isolates was used. Biofilm formation was evaluated by microtiter plates. Disc diffusion method was used for assaying bactericidal activities of antibiotics and their combinations with AgNPs against the isolates. Results: Mean MICs of AgNPs for Salmonella serotypes, E. coli, S. aureus and S. agalactiae were 3.125, 6.25, 6.25 and, 12.5 ug/mL, respectively. S. aureus and S. agalactiae showed more sensitivity (increase in fold) to examined antibiotics plus AgNPs compared to Salmonella serotypes and E. coli. The results showed that AgNPs had strong antibacterial and antibiofilm activities against the examined pathogens. Synergistic and antagonistic effects of AgNPs in combination with tetracyclin, gentamicin, streptomycin, kanamycine, cephalosporin and penicillin were observed in different cases. Conclusion: AgNPs alone or in combination with antibiotics could potentially be used as effective antibacterial and antibiofilm agents. However, our results showed that synergistic effects of antibiotics combined with AgNPs cannot be considered as a rule.