Inactivation of Giardia muris cysts by free chlorine

The chlorine resistance of cysts of the flagellate protozoan Giardia muris was examined. This organism, which is pathogenic to mice, is being considered as a model for the inactivation of the human pathogen Giardia lamblia. Excystation was used as the criterion for cyst viability. Experiments were performed at pH 5, 7, and 9 at 25 degrees C and pH 7 at 5 degrees C. Survival curves were "stepladder"-shaped, but concentration-time data generally conformed to Watson's Law. Chlorine was most effective at neutral pH and was only slightly less so in acidic solutions. Comparison of inactivation data based on equivalent hypochlorous acid concentrations, which corrects for chlorine ionization, showed that the cysts have a pH-dependent resistance to inactivation. Concentration-time (C X t') products for free chlorine obtained at 25 degrees C ranged from a low of 50 mg min/liter at pH 5 to a high of 218 mg min/liter at pH 9 and were as high as 1,000 mg min/liter at 5 degrees C. It appears that G. muris cysts are somewhat more resistant to inactivation than G. lamblia cysts and rank among the microorganisms that are most resistant to inactivation by free chlorine.