Resistance of Blastocystis to chlorine and hydrogen peroxide

Dr Anastasios Tsaousis has been working with members of the COST Action under One Health, to look at the effect of Blastocystis on the gastrointestinal tract of humans and other animals.

What is Blastocystis?

Blastocystis is a ubiquitous, widely distributed protist that inhabits the gastrointestinal tract of humans and animals.

The organism is genetically diverse, and so far, at least 30 subtypes (STs) have been identified with ST1–ST9 being the most common in humans. The pathogenicity of Blastocystis is controversial, hence its status as parasite is questionable. Several routes of transmission have been proposed including faecal-oral (e.g. zoonotic, anthroponotic) and waterborne. Research on the latter has gained traction in the last few years with the organism having been identified in various bodies of water, tap water and rainwater collection containers including water that has been previously filtered and/or chlorinated.

The team have been assessing the resistance of 11 strains maintained in culture, spanning ST1–ST9 to various chlorine and hydrogen peroxide concentrations for 24 hours and performed recovery assays along with re-exposure. Following treatment with both compounds, all subtypes showed increased resistance, and viability could be visualised at the cellular level. These results are hinting at the presence of mechanism of resistance to both chlorine and hydrogen peroxide. As such, this pilot study can be the platform for developing guidelines for water treatment processes.

This may have an impact on water treatment solutions and the beach sewage issue. Blastocystis is found in the gut of more than a billion people of all ages and gastrointestinal health statuses. The microorganism has also been found in the gut of almost every animal that has been looked at and the environment (water and soil). Nonetheless, it is still unclear how Blastocystis is acquired, though waterborne transmission has been implicated. This work shows resistance of Blastocystis to chlorine a chemical used routinely in water disinfection. This suggests that the organism can survive treatment, hence reinforcing the possibility of a water transmission route.

Dr. Anastasios Tsaousis’s  comment:

“This work adds another piece on the far from finished puzzle of Blastocystis biology and ecology reminding us an One Health approach is required to improve the health of humans, animals and the environment we all live in.”

All authors of this publication are members of the COST Action Blastocystis under One Health (, which is led by Dr. Tsaousis.