A Laboratory Study of the Synergistic Effect of Chlorhexidine and Silver

Objective:

This poster describes the results of a study that was undertaken to evaluate in vitro the antimicrobial activity of a combination of two antimicrobial agents, chlorhexidine diacetate (CHA) and silver sulphate, against five different strains of bacteria.

Methods:

A Minimum Inhibitory Concentration (MIC) assay was used to determine the effect of CHA and silver sulphate on four different strains of Pseudomonas aeruginosa (ATCC 15442, ATCC 15692 and two clinical isolates) and one strain of methicillin-resistant Staphylococcus aureus (CCUG 31966). The synergy of CHA and silver sulphate was evaluated by means of a checkerboard assay to determine the fractional inhibitory concentration index (FICI). Synergy was defined as an FICI of ≤0.5.

Results:

The checkerboard microdilution assay showed a synergistic or enhanced interaction between the paired antibacterial agents CHA and silver sulphate (i.e., FICI ≤0.5) against all bacterial strains tested. For example, in one strain of Pseudomonas aeruginosa (ATCC 15692), CHA gave an MIC value of 22 µg/ml when assayed in isolation however, combined with silver sulphate the MIC value fell to ≤0.14 µg/ml. A similar trend was seen for CHA and MRSA (0.7 µg/ml alone and reducing to 0.05 µg/ml in combination).

Conclusion:

The combination of chlorhexidine diacetate and silver sulphate has synergistic, enhanced antibacterial activity. This observation is of relevance to clinical practice, particularly in relation to the use of cover dressings incorporating antimicrobial agents.