The role of silver and sulfadiazine in the mechanism of action of silver sulfadiazine on burn wound infections was investigated. Silver, but not sulfadiazine, was bound by bacteria. Sulfadiazine did not act as an antibacterial agent in low concentrations, but exhibited specific synergism in combination with subinhibitory levels of silver sulfadiazine. The efficacy of silver sulfadiazine is thought to result from its slow and steady reactions with serum and other sodium chloride-containing body fluids, which permits the slow and sustained delivery of silver ions into the wound environs. In this circumstance, a relatively minute amount of sulfadiazine appears active.
The risk of development of resistance in Staphylococcus epidermidis to the antibiotics and antiseptics impregnated in central venous catheters was evaluated. The culture was passaged 10-20 times through subinhibitory concentrations of different antimicrobials, singly and in combination, and the MIC of each antimicrobial before and after passage was compared. There was a 10- to 16-fold increase in the MIC of the combination of minocycline and rifampicin, while no significant increase in the MIC of minocycline alone was seen. The MIC of rifampicin was 25,000-fold higher against strains passaged through rifampicin alone (as compared with that for the original strain), while the increase was only 80-fold when it was combined with minocycline for passage. There was no substantial change in susceptibility to the antiseptic chlorhexidine when used alone or in combination with either silver sulphadiazine or triclosan (the MIC of triclosan alone increased eight-fold). In time-kill studies, synergy was observed between chlorhexidine and both triclosan and silver sulphadiazine. Zone of inhibition tests of catheters impregnated with minocycline and rifampicin showed that their activity against rifampicin-resistant strains was lower than that against the susceptible strain. On the other hand, the activity of the antiseptic (chlorhexidine and silver sulphadiazine) catheters against the rifampicin-resistant and -susceptible strains was similar. Although this study indicates that antibiotic catheters may be at a higher risk of being colonized by antibiotic-resistant bacteria, the implications of these results in clinical settings need to be determined.
Catheters impregnated with synergistic combinations of chlorhexidine, silver sulfadiazine, and triclosan exhibited broad-spectrum, long-term resistance against microbial colonization on their outer surfaces.
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