The Effect of pH on the Antimicrobial Efficiency of Silver Alginate on Chronic Wound Isolates

Abstract

Nonhealing and stalled chronic wounds are often reported to reside within an alkaline environment. Consequently, a number of researchers have proposed that lowering the pH of a chronic wound environment will enable healing to progress. However, it is not known whether the efficacies of silver-impregnated wound dressings are affected by pH. To investigate whether pH has an effect on the antimicrobial barrier efficacy of a silver alginate wound dressing on wound isolates. Twenty-five bacteria and yeasts that had been routinely isolated from chronic wounds were separately exposed to a silver alginate wound dressing with the use of a standardized corrected zone of inhibition (CZOI) assay. The silver alginate dressing demonstrated a broad spectrum of antimicrobial barrier activity within the dressing against all wound isolates. However, at a pH of 5.5, compared with a pH of 7, the antimicrobial barrier activity of the silver alginate dressing significantly increased. For all yeasts the CZOI ranged from 6.25 to 11mm at a pH of 7. At a pH of 5.5, the CZOI range increased from 8.5 to 12.25mm. For the Gram-negative isolates, the CZOI ranged from 0.75 to 6.5mm at a pH 7, compared with a CZOI range of 2.75 to 8mm at pH 5.5. The CZOI for the Gram-positive isolates, including meticillin-resistant Staphylococcus aureus, ranged from 3 to 7.75mm at pH 7 and from 4.5 to 11.75mm at pH 5.5. For all isolates tested, excluding one strain of Candida albicans and one vancomycin-resistant Enterococcus strain, lowering pH to 5.5 resulted in an improvement in the antimicrobial barrier activity within the silver alginate dressing. Based on these initial in vitro findings, it is possible to suggest that there may be benefits to maintaining an infected or recalcitrant wound in a slightly acid (pH 5.5) environment. In particular, doing so may lead to an enhanced antimicrobial barrier effect of silver, a quicker reduction in the wound microbial bioburden, and therefore a reduction in the need for prolonged antimicrobial use. However, more in vitro and in vivo studies would be warranted to further substantiate these claims.