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.
