Flow cytometric analysis of bacteria- and virus-like particles in lake sediments

Flow cytometry (FCM) was successfully used to analyze freshwater bacteria and viruses in lake sediments after relatively simple sample treatment and optimization of dilution/fixation/staining procedures. Biological particles from Lakes Geneva and Bourget were first separated from the sediments by using both Sodium Pyrophosphate (0.01 M final concentration) and Polyoxyethylene-Sorbitan Monooleate (10% final concentration) and sonicating for 3 min in a water bath. The best results (based on FCM signature and the highest virus and bacterial yields from the sediments) were obtained by formaldehyde fixation carried out within less than one hour (2% final concentration, vs. no fixation or using glutaraldehyde at different concentrations), SYBR-Green II staining (×1 / 20,000 stock solution concentration, vs. use of SYBR-Gold and SYBR-Green I dyes at different concentrations). There was a considerable loss of particles after only a few days of storage at either 4 or - 22°C. For FCM analysis, the samples were diluted in Tris-EDTA buffer (pH 8) and heated for 10 min at 75°C after incubating for 5 min in the dark. The bacterial and viral counts paralleled those obtained using epifluorescence microscopy (EFM), but EFM always gave lower counts than FCM. Analysis of the distribution of the viruses in the water column and in the sediments of Lakes Bourget revealed a marked gradient, with larger quantities in the top layer of the sediment than in the water above it. These results are discussed, as well as the possible novel application of flow cytometry in the study of aquatic viral ecology.