Synergistic Effects of Gold Nanoparticles Mixed with Gentamicin, Erythromycin, Clindamycin, Bacitracin, and Polymyxin B against Staphylococcus aureus, Staphylococcus saprophyticus, Staphylococcus epidermidis, Enterococcus faecium and Enterococcus faecalis

 The majority of bacterial infections are now treatable using different classes of antibiotics. However, the world has faced a challenge called antimicrobial resistance that will diminish most antibiotics' beneficial impacts. A valuable strategy to prevent this adverse phenomenon is to increase the antibacterial effects of antibiotics using various materials as antibiotic enhancers. The aim of this project was to investigate the synergistic effects of gold nanoparticles (with a concentration of 100-200 µg/mL, a size of 16 nm, and an average zeta potential of -54.4 mV) and different antibiotics against some gram-positive cocci.

 Standard Kirby-Bauer methods were used to test the antimicrobial properties of different concentrations of gold nanoparticles mixed with MIC levels of gentamycin, erythromycin, clindamycin, bacitracin, and polymyxin B against ATTC strains of S. aureus, S. saprophyticus, S. epidermidis, E. faecium, and E. faecalis.

It was indicated that the 25:75 ratio of AuNPs with gentamicin led to a larger zone of inhibition against S. aureus, S. epidermidis, and E. faecalis compared with pure antibiotics. Moreover, this increase was found against E. faecalis when applying 25:75, 50:50, and 75:25 ratios of AuNPs with clindamycin. Similarly, an increase in the diameter of the zone of inhibition against S. epidermidis was observed when using 25 μL AuNPs with 75 μL bacitracin. Additionally, a synergistic antibacterial effect against S. saprophyticus was found when using AuNPs and polymyxin B with a ratio of 50:50.

 It was concluded that suitable concentrations of gold nanoparticles could enhance the antibacterial activities of antibiotics.