NUCLEIC-ACID STAINS AS INDICATORS OF GIARDIA-MURIS VIABILITY FOLLOWING CYST INACTIVATION

A reliable viability assay for Giardia is required for the development of disinfection process design criteria and pathogen monitoring by wa ter treatment utilities. Surveys of single-staining nucleic acid dyes (stain dead parasites only), and double-staining vital dye kits from M olecular Probes (stain live and dead parasites) were conducted to asse ss the viability of untreated, heat-killed, and chemically inactivated Giardia muris cysts. Nucleic acid staining results were compared to t hose of in vitro excystation and animal infectivity. Nucleic acid stai n, designated as SYTO(R)-9, was considered the best among the single-s taining dyes for its ability to stain dead cysts brightly and its rela tively slow decay rate of visible light emission following DNA binding . SYTO-9 staining was correlated to animal infectivity. A Live/Dead Ba cLight(TM) was found to be the better of 2 double-staining viability k its tested. Logarithmic survival ratios based on SYTO-9 and Live/Dead BacLight were compared to excystation and infectivity results for G. m uris cysts exposed to ozone or free chlorine. The results indicate tha t SYTO-9 and Live/Dead BacLight staining is stable following treatment of cysts with chemical disinfectants. Copyright (C) 1996 Australian S ociety for Parasitology.