Characterization of culturability, protistan grazing, and death of enteric bacteria in aquatic ecosystems.

Nonstained bacteria (NSB), rhodamine-stained bacteria (RSB), and fluorescence-labeled bacteria (FLB) were prepared from two enteric bacterial species, Escherichia coli and Enterococcus faecalis. Counts of CFU of NSB and RSB and total numbers of RSB and FLB were monitored over time, both in the presence and in the absence of natural microbiota. In the presence of natural microbiota, no differences were observed between CFU counts of NSB and RSB, but RSB total numbers were 1 to 4 orders of magnitude higher than CFU numbers. Therefore, the use of standard bacteriological media causes an important underestimation of the total number of enteric bacteria. In the absence of natural microbiota, the total numbers of NSB, RSB, and FLB remained constant over time. These results showed that RSB are a reliable indicator of the decay in both the total number and the CFU of enteric bacteria in natural water samples. By using RSB, enteric bacteria were classified as culturable cells, nonculturable cells (or somnicells), and dead cells in the presence of natural microbiota. In the presence of natural microbiota, differences between RSB and FLB direct counts were detected for E. coli, but not for E. faecalis. These differences were explained by size-selective grazing. Thus, protistan grazing was found to be the main cause of the decrease in total numbers of enteric bacteria in our experiments.