The ideal biocide

In the last twenty or more years chlorine dioxide (ClO2) emerged as a new and popular inorganic disinfectant. It is often referred to as „the ideal biocide” because of its advantageous properties. In spite of that, as far as we know, ClO2 solutions are not frequently used as antiseptic. This is because the available ClO2 solutions were more or less contaminated with other chemicals applied in its synthesis and that contamination formed a major obstacle in medical applications like treating infected wounds, for example. Since 2006, however, with the help of an invention, it is relatively easy to produce high purity aqueous ClO2 solutions. These solutions are already commercially available and have been successfully used in dentistry since 2008. Thus, it seems reasonable to ask the question whether the “ideal biocide” in its pure form can also be an “ideal local antiseptic” at the same time?

How is it possible that contacting or even drinking ClO2 solution is practically harmless for animals and human beings, while the same aqueous solution can be a very effective and a rapid killer for bacteria, fungi, and viruses? What is the basis of this unexpected selectivity?

The answer is the following: the selectivity between humans or animals and microbes is based not on their different biochemistry, but on their different size. Denominating ClO2 in the title as a „size selective” antimicrobial agent aims to emphasize this new type of selectivity. It was found that the characteristic time necessary to kill a microbe is only a few milliseconds. As ClO2 is a rather volatile compound its contact time (its staying on the treated surface) is limited to a few minutes. While this stay is safely long enough (being at least 3 orders of magnitude longer than the killing time) to inactivate all bacteria on the surface of the organism, it is too short for ClO2 to penetrate deeper than few tenths of a mm; thus, it cannot cause any real harm to an organism which is much larger than a bacterium.

ClO2 is a strong, but a rather selective oxidizer. Unlike other oxidants it does not react (or reacts extremely slowly) with most organic compounds of a living tissue. ClO2 reacts rather fast, however, with cysteine and methionine (two sulphur containing amino acids), with tyrosine and tryptophan (two aromatic amino acids) and with two inorganic ions: Fe2+ and Mn2+. It is generally assumed that the antimicrobial effect of ClO2 is due mostly to its reactions with the previously mentioned four amino acids and their residues in proteins and peptides. In the peptide group it is important to mention glutathione – a small tripeptide containing cysteine – which is a major antioxidant in cells, with an intracellular concentration of 0.1-10 mM.

We hope the study can initiate clinical studies that ClO2 could be applied to treat various local infections, especially where bacterial resistance is a problem.