How Chlorine Dioxide Damages Bacterial Cells: Thiol Oxidation and Oxidative Stress

A selective oxidant, not a chlorinating agent

Chlorine dioxide is a dissolved free radical that reacts by taking single electrons from susceptible molecules. Unlike hypochlorous acid, it does not add chlorine atoms to organic material, which is why it does not form trihalomethanes. Its reactivity is selective: it attacks reduced sulphur groups (thiols such as cysteine and glutathione), the amino acids tyrosine and tryptophan, and some other electron-rich sites, while being comparatively unreactive towards many carbohydrates and saturated hydrocarbons.

Because thiol groups sit in enzyme active sites, membrane transport proteins and the cell's own antioxidant buffer, oxidising them disrupts several functions at once: enzymes lose activity, membranes become leaky, protein synthesis falters and the cell's capacity to repair oxidative damage is consumed. Loss of membrane integrity and leakage of cell contents are commonly reported endpoints in chlorine dioxide inactivation studies.

The 2026 Aeromonas study

A study by Cao and colleagues, available online in Fish & Shellfish Immunology on 3 September 2026 (in press; doi 10.1016/j.fsi.2026.111707), examined chlorine dioxide against bacterial pathogens of farmed rainbow trout. Chlorine dioxide markedly inhibited Aeromonas hydrophila and Aeromonas salmonicida and reduced the thiol content of the bacterial cells in a concentration-dependent manner, which the authors interpret as evidence that thiol-associated oxidative damage contributes to its antibacterial action.

The same study exposed live fish to chlorine dioxide. Low-to-moderate exposure activated antioxidant and innate immune responses, whereas higher concentrations caused mortality, liver and gill injury, depletion of glutathione (the cell's main thiol antioxidant), lipid peroxidation and immune disturbance. Transcriptome analysis of infected fish treated with chlorine dioxide showed changes in glutathione metabolism, xenobiotic metabolism and immune signalling pathways.

Dose-response: the same chemistry cuts both ways

The study's most useful lesson for water and hygiene practice is that the thiol chemistry that inactivates bacteria is not specific to bacteria. Host cells, including those of animals drinking or living in treated water, use the same glutathione system. Antibacterial benefit and host toxicity therefore sit on one dose-response curve, and the usable window between them depends on organism, exposure route, contact time and water chemistry. This is one reason biocidal product authorisations specify concentrations, exposure scenarios and species rather than leaving dosing open.

Oxidant demand also matters. Chlorine dioxide is consumed by organic matter, biofilm and some materials before it reaches target cells, so the concentration applied and the concentration actually experienced by bacteria can differ substantially, as the dental waterline guide in this centre shows.

What this study does and does not establish

It establishes, in an aquaculture laboratory setting, that chlorine dioxide inhibits two Aeromonas species and depletes bacterial thiols in a concentration-dependent way, consistent with the oxidative mechanism described above, and that fish show a dose-dependent redox response with injury at higher exposures.

It does not establish the efficacy of any ChloroKlean product, which was not tested. It does not provide a commercial dose for aquaculture, livestock, veterinary or water-system use; the concentrations were chosen for a mechanistic experiment and a toxicological boundary in one fish species. It is not a field trial, and the paper was in press at the time of writing, so figures may change in the final version. Any veterinary hygiene or drinking-water use must follow the product's authorised uses, label and a competent adviser or veterinarian, not this study.

Evidence and uncertainty

Published biofilm studies are valuable for understanding mechanisms, but their conditions may not match a particular installation. Species, surfaces, deposits and operating conditions should be recorded when interpreting evidence.

For safety-critical systems, decisions should be documented through the relevant risk assessment and management plan.

Choosing the next question

A useful next step is to identify what is known, what is inferred and what needs verification. This avoids treating a general reference as a site diagnosis.

Where a product is considered, confirm the intended use, authorisation and label directions independently of this educational guide.

Use this sequence to frame investigation and control; it is not a dosing protocol.

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