Chlorine dioxide is a size-selective antimicrobial agent

Abstract

Background / aims: ClO2, the so-called "ideal biocide", could also be applied as an antiseptic if it was understood why the solution killing microbes rapidly does not cause any harm to humans or to animals. Our aim was to find the source of that selectivity by studying its reaction-diffusion mechanism both theoretically and experimentally.

Methods: ClO2 permeation measurements through protein membranes were performed and the time delay of ClO2 transport due to reaction and diffusion was determined. To calculate ClO2 penetration depths and estimate bacterial killing times, approximate solutions of the reaction-diffusion equation were derived. In these calculations evaporation rates of ClO2 were also measured and taken into account.

Results: The rate law of the reaction-diffusion model predicts that the killing time is proportional to the square of the characteristic size (e.g. diameter) of a body, thus, small ones will be killed extremely fast. For example, the killing time for a bacterium is on the order of milliseconds in a 300 ppm ClO2 solution. Thus, a few minutes of contact time (limited by the volatility of ClO2) is quite enough to kill all bacteria, but short enough to keep ClO2 penetration into the living tissues of a greater organism safely below 0.1 mm, minimizing cytotoxic effects when applying it as an antiseptic. Additional properties of ClO2, advantageous for an antiseptic, are also discussed. Most importantly, that bacteria are not able to develop resistance against ClO2 as it reacts with biological thiols which play a vital role in all living organisms.

Conclusion: Selectivity of ClO2 between humans and bacteria is based not on their different biochemistry, but on their different size. We hope initiating clinical applications of this promising local antiseptic.

Figure 1. Apparatus to measure ClO₂ transport through gelatine or pig bladder membranes.

The two glass parts of the apparatus are held together by a pair of extension clamps (not shown in the Figure) which are fixed to a support stand by clamp holders. The active cross-section of the membranes is 28 cm². See text for the working principle.

Figure 2. Permeation of ClO₂ through a gelatin membrane as a function of time t.

Each point in the diagram represents a „black burst” (see Methods). V is the cumulative volume of the 0.01 M Na₂S₂O₃ titrant added before the burst and N is the amount of ClO₂ permeated until time t. T _L1 = 627 s and T _L2 = 175 s are time lags of the first and the second experiments, respectively. The concentration of ClO₂ source in the magnetically stirred aqueous solution was 1360 ppm (mg/kg) or 20.1 mM.

Figure 3. Permeation of ClO₂ through a pig bladder membrane as a function of time t.

V and N have the same meaning like in Figure 2. T _L1 = 2770 s, T _L2 = 586 s and T _L3 = 226 s are time lags of the experiments performed on the 1st, 2nd, and 3rd day, respectively. The concentration of the ClO₂ source was 946 ppm (14.0 mM) in these experiments.