PROTOZOAN BACTERIVORY AND ESCHERICHIA-COLI SURVIVAL IN DRINKING-WATERDISTRIBUTION-SYSTEMS

The development of bacterial communities in drinking water distributio n systems leads to a food chain which supports the growth of macroorga nisms incompatible with water quality requirements and esthetics, Neve rtheless, very few studies have examined the microbial communities in drinking water distribution systems and their trophic relationships. T his study was done to quantify the microbial communities (especially b acteria and protozoa) and obtain direct and indirect proof of protozoa n feeding on bacteria in two distribution networks, one of GAC water ( i.e., water filtered on granular activated carbon) and the other of na nofiltered water. The nanofiltered water-supplied network contained no organisms larger than bacteria, either in the water phase (on average , 5 x 10(7) bacterial cells liter(-1)) or in the biofilm (on average, 7 x 10(6) bacterial cells cm(-2)). No protozoa were detected in the wh ole nanofiltered water-supplied network (water plus biofilm), In contr ast, the GAC water-supplied network contained bacteria (on average, 3 x 10(8) cells liter(-1) in water and 4 x 10(7) cells cm(-2) in biofilm ) and protozoa (on average, 10(5) cells liter(-1) in water and 10(3) c ells cm(-2) in biofilm), The water contained mostly flagellates (93%), ciliates (1.8%), thecamoebae (1.6%), and naked amoebae (1.1%). The bi ofilm had only ciliates (52%) and thecamoebae (48%), Only the ciliates at the solid-liquid interface of the GAC water-supplied network had a measurable grazing activity in laboratory test (estimated at 2 bacter ia per ciliate per h), Protozoan ingestion of bacteria was indirectly shown by adding Escherichia coli to the experimental distribution syst ems, Unexpectedly, E, coli was lost from the GAC water-supplied networ k more rapidly than from the nanofiltered water-supplied network, perh aps because of the grazing activity of protozoa in GAC water but not i n nanofiltered water, Thus, the GAC water-supplied network contained a functional ecosystem with well-established and structured microbial c ommunities, while the nanofiltered water-supplied system did not, The presence of protozoa in drinking water distribution systems must not b e neglected because these populations may regulate the autochthonous a nd allochthonous bacterial populations.