Supathra Tiewchaloren, Virach Junnu 
Department of Parasitology, Faculty of Medicine Siriraj Hospital, Mahidol University, 2 Prannok Road, Bangkoknoi, Bangkok 10700, Thailand 

Abstract 
Three pathogenic Naegleria species were compared to a reference strain for their ability to withstand extremes of temperature, pH, salinity, chlorine, ultraviolet light and formalin. There was no marked difference between the strains. Trophozoites degenerated in hours at temperatures below -40°C. After being frozen, trophozoites in both groups did not survive in culture in axenic Nelson medium. Pathogenic Naegleria sp. were killed at temperatures greater than 50°C for 24 hrs. Amebae could not survive in a pH of less than 5 or more than 12. Trophozoites were destroyed in saline concentrations higher than 2% and chlorine concentrations greater than 0.75 ppm. In addition, all Naegleria were sensitive to formalin at concentrations greater than 0.05%. However, they were resistant to ultraviolet light.