Abstract
Purpose: :
Previous pharmacodynamic studies have demonstrated clear quantitative relationships between total hydrogen peroxide exposures (expressed as ppm peroxide x hr of exposure) and microbial killing (expressed as log reduction values) for ISO/FDA specified bacteria, yeast, and mold tested at constant peroxide concentrations in test tubes. Here, pharmacodynamic analysis was used to instead evaluate efficacy of a model one-step peroxide lens disinfection system wherein microbial killing and peroxide neutralization occur simultaneously in lens cases containing platinum neutralizing disks.
Methods: :
A series of phosphate buffered solutions (pH 6.5, ~250 mOsm) containing 1.0 - 5.0 % peroxide (10,000 - 50,000 ppm) were challenged with Serratia marcescens (Sm) prepared in 10% organic soil in one-step neutralizing lens cases per the ISO 14729 standard for disinfection efficacy testing. Inoculated lens cases were sampled at 10, 20, 30, 60, and 120 min for parallel enumeration of surviving bacteria (to determine log reduction values) plus colorimetric redox titrations of residual peroxide concentrations (to determine total peroxide exposures as ppm x hr).
Results: :
Sm log reduction values ranged from 0.0 for 10 min treatment with initial 1.0% peroxide to >4.8 for any treatment ≥ 10 min with initial 5.0% peroxide. Corresponding total peroxide exposures ranged from 1,168 - 7,609 ppm x hr. For all time points, complete killing (i.e. >4.8 log reduction) required ~5,000 ppm x hr, intermediate killing (i.e. 3.4 log reduction) required ~3,700 - 4,300 ppm x hr, and reduced killing (i.e. 1.6 - 1.8 log reduction) required ~2,400 - 2,800 ppm x hr peroxide exposures. The R2 curve fit value for measured peroxide exposures versus Sm log reductions in lens cases was 0.98.
Conclusions: :
Pharmacodynamic analysis of Sm killing conducted in one-step peroxide neutralizing lens cases demonstrates that there remains a clear quantitative relationship between total hydrogen peroxide exposures and microbial killing, even when microbial killing and neutralization of active peroxide occur simultaneously. For Sm in lens cases, short treatments with high peroxide concentrations are quantitatively equivalent to longer treatments with lower peroxide concentrations.
Keywords: keratitis • contact lens • cornea: basic science