Bacteria which are resistant to antibiotics are a growing concern in healthcare. The overuse of antibiotics has contributed to the rise of so-called “superbugs”, which are resistant to multiple antimicrobials. New treatment options for such infections are urgently needed. One approach is combining antibiotics with silver nanoparticles (AgNPs), which have an antimicrobial effect. The combination of the two can synergistically inhibit bacterial growth, i.e., the nanoparticles can enhance the effect of the antibiotic. This finding has not been fully understood.

Hongtao Yu and colleagues, Jackson State University, MS, USA, have studied the mechanism of this synergy for combinations of silver nanoparticles (AgNPs) with different antibiotics: ampicillin, penicillin, enoxacin, kanamycin, neomycin, and tetracycline. They tested the effect on the bacterium Salmonella typhimurium DT 104, which is resistant to all six of these antibiotics. The team found that the antibiotics enoxacin, kanamycin, neomycin, and tetracycline show synergy with AgNPs in inhibiting bacterial growth, while ampicillin and penicillin do not. The inhibition (up to 100 %) was much greater than for AgNPs alone or the antibiotic alone.

The researchers used UV−Vis and Raman spectroscopy to understand this effect. They found that only the “synergistic” antibiotics can form complexes with AgNPs. The proposed mechanism involves the complex formation between antibiotic and AgNPs, the binding of these complexes to the bacteria, and the release of a large amount of Ag⁺ ions close to the bacteria, which causes cell death.

• Mechanistic Study of the Synergistic Antibacterial Activity of Combined Silver Nanoparticles and Common Antibiotics,
  Hua Deng, Danielle McShan, Ying Zhang, Sudarson S. Sinha, Zikri Arslan, Paresh C. Ray, Hongtao Yu,
  Environ. Sci. Technol. 2016.
  DOI: 10.1021/acs.est.6b00998