21st European Congress of Clinical Microbiology and Infectious Diseases

Antibacterial mode of action and resistance studies with the silver cation (Ag+) in Staphylococcus aureus

Abstract number: P1454

Objectives: Compounds containing Ag+ are commonly used to treat and prevent bacterial infection in burns and chronic wounds. However, the antibacterial mode of action of Ag+ remains poorly characterised. Furthermore, the current susceptibility levels of clinical isolates towards Ag+ and the propensity for bacteria to develop Ag+ resistance have not been established. Here we present a series of studies to investigate these aspects in Staphylococcus aureus.

Methods: Membrane integrity of S. aureus SH1000 exposed to Ag+ (in the form of silver nitrate) at 4x the minimum inhibitory concentration (MIC) was examined using the BacLight^® assay, and by measurement of potassium ion (K+) leakage from cells using atomic absorption spectroscopy. Bacteria were examined for morphological changes using scanning electron microscopy (SEM) after exposure to Ag+ at 4x MIC for 5 and 120 min. Silver nitrate MICs were determined by agar dilution for a collection of S. aureus isolates (n = 833) collected from hospitals across Europe between 1997 and 2010. To examine the in vitro development of staphylococcal resistance to Ag+, S. aureus SH1000 was subjected to continuous subculture in the presence of sub-MIC concentrations of Ag+ for up to 42 days.

Results: Membrane integrity decreased by 97% after 10 min exposure to Ag+ as determined by the BacLight^® assay, coinciding with 96% loss of intracellular K+. SEM revealed no morphological changes in bacteria exposed to Ag+. All clinical S. aureus isolates were susceptible to 8–16 mg/L silver nitrate. Reduced susceptibility to Ag+ was not observed in S. aureus SH1000 following 42 days continuous challenge with sub-MIC concentrations of Ag+.

Conclusions: Although no overt cell lysis occurred in cells following challenge with Ag+, the rapid and extensive loss of membrane integrity observed strongly suggests that the antibacterial activity of Ag+ results directly from damage to the bacterial membrane. The universal susceptibility of staphylococcal isolates to Ag+, coupled with the inability to select endogenous resistance upon extended exposure to Ag+, confirm that silver compounds remain a viable alternative to antibiotics for the treatment of topical staphylococcal infections.