DIFFERENTIAL ELIMINATION OF ENTERIC BACTERIA BY PROTISTS IN A FRESH-WATER SYSTEM

The short-term (1 h) and long-term (3 d) elimination of low and high d ensities of five enteric bacteria, Klebsiella pneumoniae, Aeromonas hy drophila, Escherichia coli, Enterococcus faecalis and Staphylococcus e pidermidis, by flagellate and ciliate protists were measured in a fres hwater system. In addition, the two processes, ingestion and digestion , which cause the disappearance of those enteric bacteria as time pass es, were quantified. The results showed that the elimination of these enteric bacteria by protists depends on their initial density, which c onfirms that the lower the bacterial density the more difficult is the ir elimination. On the other hand, the short-term and long-term elimin ation rates of each enteric bacteria were different, and moreover, the order of priority for elimination in the two cases was not the same. Escherichia coli showed the highest elimination rate in short-term exp eriments, while Aer. hydrophila disappeared at highest rates in long-t erm experiments. This different order of priority in the elimination r ates and the different digestion rates on the five enteric bacteria by phagotrophic protists indicated that the elimination in time is very much influenced by the digestive capacity on each enteric bacteria of those protists. Thus, the low digestion rates of Ent. faecalis and Sta ph. epidermidis by flagellates and ciliates as well as their low disap pearance percentages in the long-term experiments confirm that enteric Gram-positive bacteria are eliminated from the aquatic systems at low er rates, because their digestion is difficult.