ACID-FAST BACTERIA AND YEASTS AS INDICATORS OF DISINFECTION EFFICIENCY.

Since the coliform group of organisms is considered to be less resistant to chlorine than some bacterial and viral pathogens, the utility of both yeast and acid-fast organisms as potential indicators of disinfection efficiency was evaluated. In most laboratory studies these two groups of organisms were represented by Candida parapsilosis and Mycobacterium fortuitum, respectively. The relative resistance of the test organisms to free chlorine was: acid-fast greater than yeast greater than coliforms. The increased chlorine resistance of these organisms appeared to be the result of the thickness and impermeability of the cell wall. It was concluded that the primary mode of action of chlorine in disinfection was disruption of the cell membrane with a resultant change in cell permeability and physical damage to the cell DNA. Resistance to ozonation was also studied. Variations in pH between 5 and 10 did not significantly affect organism survival of either yeasts or acid-fast organisms using constant ozone residuals, while increasing temperatures increased the inactivation of both organisms. Large volume sampling and enuration techniques were developed for the yeasts and acid-fast organisms using membrane filtration which enabled the enumeration of these organisms at the relatively low densities found in finished drinking water.